Equine Color Genetics By Lynn Weber

There are two base colors, black and chestnut. Everything else is a dilution, modifier, white pattern, or unexplained markings.

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Modifiers

Bay - aka Agouti, restricts black to points

Gray - changes base coat over horse's lifetime by gradually adding white hairs throughout (much like a person's hair turns gray)

Flaxen - lightens mane and tail to a blonde color, only on chestnut based colors

Sooty/Smutty - darkens overall color, often has dapples

Mealy/Pangare - lightens nose, flanks, elbows, belly, can be very extensive, or not much

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Dilutions

Champagne - dilutes both black and red based colors, eyes tend to lighten and skin is pink and tends to freckle
  - Black is called Classic Champagne
  - Bay is Amber Champagne
  - Chestnut is Gold Champagne

Cream - dilutes red pigment, may lighten black a little in single form, in a double dose the smoky crème looks a lot like a perlino, this gene intensifies it's effect in homozygotes
  - Black (x 1=smokey black, x 2=smoky creme)
  - Bay (x 1=buckskin, x 2=perlino)
  - Chestnut (x 1=palomino, x 2=cremello)

Dun - dilutes body color, but not points. Will have a dorsal marking of a darker color down the back, as well as striping on the legs. Sometimes "cobwebbing" occurs on the face, and barring on the withers. Different shades have different names, but all are variations of the below.
  - Black becomes grulla/grullo
  - Bay becomes dun
  - Chestnut becomes red dun

Silver - dilutes black only, lightens mane and tail more than body color in most cases, bays will tend to have flaxen manes and not quite black points, chestnut does not look any different if it has the gene. Sometimes called "Chocolate" or "Silver Dapple", although horses are not always dappled.

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White Markings

Non-Pinto Patterns
Appaloosa - a lot is still unknown about this/these genes. Snowcaps and few-spots are believed to be homozygous

Roan (also called Classic or True Roan)- adds white hair to body, but not face and points

Rabicano - roaning along the flank, "skunk tail" roaning at base of tail

Pinto Patterns
Mixed Patterns - many overos (frame, sabino, splash) have more than one pattern, and tobianos can also have any or all of the three overo patterns

Frame - characterized by white on the sides of the body, not crossing over the back (but can cross over the neck), not much leg white. Lethal (LWO - Lethal White Overo) when the gene is inherited from both parents, when breeding pintos of any breed get them tested to see if they have it and if they do, don't breed to a horse that also has it

Sabino - leg white, facial white, chin white, belly white. Some speculate that all white markings, a simple star, or a sock, are related to sabino

Splashed White - looks like horse has been dipped in white paint, face will be white, white coming up legs onto body, blue eyes

Tobiano - dark head, almost always 4 white feet/legs, white crosses over back

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Unexplained Markings

Bend Or Spots - dark spots randomly on the coat, not from scaring or any other external cause

Birdcatcher Spots - white spots randomly on the coat, *may* have something to do with sabino, they are still looking into that

Brindle - very rare vertical striping

Lacing - lacy white markings on the horse's back in a reticulated pattern like a giraffe's

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References for Equine Color Genetics Information

Websites:
Equine Color Genetics: Studies, Deb Bennett, PhD
Horse Color Articles by Lesli Kathman

Recommended Books:
Equine Color Genetics (Second Edition) by Dr. Philip Sponenberg
Equine Color Genetics by Dr. Philip Sponenberg
Horse Color by Dr. Philip Sponenberg amd Dr. Bonnie V. Beaver
Horse Color Explained by Jeanette Gower