Let me preface this by saying this is going to be long, perhaps more information than you are looking for.  However, it may help you to understand the genetics of horse color and how they display.  So grab a cup of coffee and a snack.  :-) 

First off, there are only two base colors in horses, red & black. Every other shade of color is caused by a modifier.  There are several modifiers.  These modifiers can be combined making it even MORE of a challenge to determine what is going on genetically.  It is important to realize that some are dominant meaning at least one parent must carry the gene in order to produce a horse of that color.  In most cases, the parent will show the modifier.

You will also see me referring to "points", this means mane, tail, legs, ear tips, the area surrounding the eyes and muzzle.  White markings can be related to specific pattern types, or they can just be markings.  I will try to go into those a small amount.  I will be honest and say that I'm still learning on that side of things.  Since I raise Morgans, those patterns, excluding Sabino, are not generally part of the equation.

In color genetics they have assigned some modifiers with letters to identify them and help chart out the horses color.  Like E = black, e = red.  Chestnut is what some call a dominant recessive meaning the red gene is recessive, but when the horse has two of the genes, then it is dominant.            

EE = homozygous black - can NOT produce chestnut.

Ee = heterozygous black - can produce chestnut only if bred to a horse that is also heterozygous black or chestnut based.

ee = chestnut - can produce black only if bred to a horse that is homozygous or heterozygous black based.

Agouti -

This modifier is also dominant and comes in two forms, bay or brown.  The agouti modifier affects black hair over the body., not the points.  A bay or brown horse is a black based, however the agouti gene modifies the body color to a lighter shade.  All points are black.

Bays & browns come in varying shades.  A chestnut can carry agouti yet not show it since it does not modify red hair.  I will also note that bay is dominant over brown.  So a horse can have two agouti genes, one of each type yet look bay.  However, this same horse can produce a brown.  The genetic test at this point can only tell you if you have agouti, but NOT which form.

 

Cream -

This gene will lighten all hair colors EXCEPT black.  A bay or brown horse is a black based, however there is an agouti gene that modifies the body color to a lighter shade.  Cream will then modify that body color to a golden based color resulting in buckskins of various shades.  It is possible to have an extremely dark buckskin, we see these in Morgans.             The theory is that they are brown based.  They have been tested & proven to have a cream gene.

A chestnut with a cream gene will be palomino of various shades.  A true black will not show a cream gene as there is no agouti modifier, yet it can carry the cream gene and produce a cream based horse.  Such a horse is typically called a smoky black.  A buckskin can "pretend" to be a dun sporting a "false" dorsal stripe.  Yet it is not a true dun.  Keep reading for more details.

With cream homozygous horse, it really affects the color of the horse.

Perlino -

A bay or brown based horse with two cream genes.  These horses are a very pale color with points having a reddish tint & blue eyes.

Cremello -

A chestnut based horse with two cream genes. These horses are a very pale color with blue eyes, points are the same color as the body.

Smokey Cream -

A black based double dilute, these horses are affected by the modifier.  It seams when you double the cream modifier then it affects the black hair color.  They can appear to look a very similar to the perlino, or with the whole body having a slight reddish hue.  The only way to decisively prove which they are is through genetic testing for the agouti gene.

 

Silver Dapple -

This modifier will lighten ALL black pigment, while it does NOT affect red pigment.             Chestnut silver dapples do not look any different than any other chestnut based horse, however the horse can produce silver dapples if bred to a non-chestnut horse.  We refer to these horses as carriers.  On a black based horse, it can lighten up the body, mane & tail color to the point that the horse appears flaxen chestnut and is often misidentified as such.

By using genetic testing for base color you can determine if the horse is indeed a true silver dapple or just a flaxen chestnut.  You can also determine this if you breed said horse to a chestnut based horse and produce a black based horse.

 

Dun -

Ah ha! Perhaps, the most misunderstood modifier.   These horses are commonly misidentified and confused with buckskins.  Some registries even have the colors lumped together or described incorrectly.  While the two may look very similar, there are some big differences.  This modifier is also dominant & appears to modify both red & black based horses.  This is one that doesn't hide.  You will find if you compare a buckskin & dun side by side that there is a difference in the "tone" of the hair.

Buckskins appear shiny where as duns are "flat".  Much like the difference between glossy or flat paint.  A true dun will have "primitive markings".  This means a dorsal stripe which extends through the tail length, leg barring, a dark patch or stripe across the withers, spider web type markings on the face, throat & chest.  Their points are also dark, the color matching that of the primitive marks.  A horse may have all or some of these markings and they may appear lighter on some horses.  Duns come in a variety of shades, some are classified into different categories.  For some really cool examples of these colors, great photos of true dun markings vs. false markings, check out Dun Central Station.

Grulla -

Black based dun with no agouti modifiers, body appears a blue/purplish grey with black or near black points & markings.

Red Dun -

Red based dun, body appears pale red with darker red points & markings.

Bay/Brown Dun or Zebra Dun -

Black based dun with agouti modifier, these show the closest to the typical dun coloring.  Body a flat golden shade with brown/chocolate points & markings.

Cream & dun can be combined to create even more shades.  The above site will explain & show that much better than I can.  A homozygous dun does not appear to look any different, but all foals will be dun based.

 

Grey -

This modifier is dominant.  You must have a grey parent to produce a grey foal.  Foals are typically born dark then fade over time to grey.  Depending on the other modifiers at work how they look as they turn grey can be totally different.  But, they will all end up at the same place in the end, grey.  However, a speckled/flea bitten grey actually seems to gain more of the freckles/spots as they age.  A homozygous grey does not look any different, however all foals from this horse will be grey.

 

Roan -

This modifier is dominant.  To produce a roan one of the parents must also be a roan.  How a roan appears in winter coat can look utterly different than the summer coat.  A "typical" roan should have a dark head, forelock, mane, tail & legs.  The body color should be a washout of the base color.  Meaning a red (strawberry) roan would have a white body with red hairs scattered throughout.  Not in freckles, but even through the coat.

A blue roan takes on an almost blue appearance as the black hairs show through the white that way.  A homozygous roan does not look any different, however all foals from this horse will be roan.

When you mix these modifiers, you can get quite a mix of colors displayed.   Within the Minis & other pony based breeds, these modifiers show much stronger than they do within some breeds.  Also, within the Mini gene pool, modifiers are so intercrossed that it can be hard to truly determine what is going on.  As a friend of mine says, with Minis, all color bets are off.

So, here's a look at how a combo of modifiers can work on a black based horse.            

Straight bay -

The base coat of the horse is black with an agouti modifier to turn the body color red, but all points are black.

Straight buckskin -

The base coat of the horse is black with an agouti modifier to make it bay.             Then you add the cream gene to make the body golden in color, but all points are chocolate/black.            

Straight black silver dapple -

The base coat of the horse is black, with NO agouti modifier.  Body color is diluted to a chocolate shade with all points diluted to various shades.  These points can appear anywhere from white, to just lighter than the body color, to straw colored.

Straight bay/brown silver dapple -

The base coat of the horse is black, with an agouti modifier.  Body color is diluted to a lighter shade with all points diluted just like the black silver dapple.            

Silver dapple buckskin -

The base coat is black, with an agouti modifier changing the body color to red.  The cream modifier then changes the body color to a shade of gold.  The silver dapple modifier then dilutes the black points to a chocolate shade with typical lighter edges.  The mane & tail for example could look chocolate color with a varied degree of yellow to white tips to all the hair.  Again, there are always varying degrees that any of these modifiers will show.

Ok, that leaves us with two more coat color modifiers.

Sooty -

This modifier gives the horse a sooty or dirty look over their base coat.  Most commonly seen on bay, buckskin & palomino.

 

Flaxen -

This modifier works on the points, causing them to be lighter than the body color.   This modifier does NOT show on black based horses.  Some feel you must have the flaxen gene from both parents to get a flaxen foal.  Meaning it must be homozygous to show, just like chestnut.

Now there are two other factors that change how the coat looks.  These may not be true modifiers.  So I have not listed them as such.  There is still a lot in question when it comes to understanding these two.  I'm going by general consensus in my definitions.

Dapples -

It seems to be a mix of genetics & health.  Some horses are just more prone to showing off the dapples.  The healthier they are, the more the dapples will show.  Horses with a cream gene seem to show them easier.  However, I have some bays here that show them off pretty dern well.

 

 

Metallic Sheen -

I'm still working on this one a bit more.  I have seen it referred to with a term.  I'm unable to recall it, or find it at the moment.  However, I've been told that within the Akhal Teke breed the metallic effect is caused by a clear hair shaft that reflects light,  and can appear in any color coat.  So instead of a color gene, the metallic effect would appear to be caused by a gene that affects the hair structure.

Within Morgans, it seems to appear more with the cream gene, but this could just make it easier to see as I've seen it on chestnuts as well.  I actually have a red chestnut who's hair takes on a golden hue when it catches the light right.

Pinto/Paint -

There are several types and a horse can display all at once.  The color of the horse can be any shade, yet have these patterns show.  Tobiano, Overo, Tovero & Splash are the main ones.  You can find detailed information on how these display online at various horse color sites.  I might also suggest looking at the paint/pinto registries as well.

Sabino -

Dictates the amount & type of white markings on the face, legs & body.  These can be in varying degrees from just a horse with a wild face marking to a horse that is almost pure white.  Some feel this is classified as a pinto variation.  Generally a sabino will have an irregular or wildly shaped facial marking.  Often extending over the nostrils and down to the lower lip.  Sometimes you will see a spot on the lower lip without white on the upper lip.

The leg markings often have roaned edges and extend upwards to a point , the rear have the point on the front of the leg with the front legs showing the point on the back side.  With these horses you may also find a salt & pepper mane where there is white hairs mixed throughout without the rest being any lighter than the body shade.  You may have white markings elsewhere on the body, belly spots are common.

Appaloosa -

Dictates a variety of white color patterns on any hair color.  From a white horse with colored spots, to a colored horse with only a few patches of white here and there.  Some with defined spots of color on the rear in a blanket of white, or a blanket of white on the rear with irregular edges with possible spots with irregular edges inside the blanket area.

Some appaloosa based horses will gradually loose all color and turn "white".  Many times the darker skin spots will still show through the hair coat, especially when they are wet.  Also, these horses tend to have mottled skin coloration around the muzzle, eyes & private areas.