alad, alath: variegated, piebald, pinto. Similar words are “breac” (speckled, variegated, patterned) and “riabach” (brindled in cows and horses, maybe not dogs).
buidh: tan, if it’s describing fur or human hair color. (Yellow as a regular color. “Fionn,” bright, is what you call a blond person’s hair. Fair skin is also “fionn.”)
donn: brown. (Always refers to hair color, whether a dog or a human. A brown man or a brown girl has brown hair, not brown skin.)
dubh: black (Also a hair color thing, for dogs or humans. A black man has black hair, not black skin. A black- or brown-skinned man is described in Irish as having skin that is “gorm,” a dark/low-intensity blue or green.)
liath: gray, especially fur and hair. (Gray eyes were traditionally considered “uaine,” which usually means a bright green, but in this case it’s a bright or vibrant/shiny gray. “Glas” is yet another word for green, but it can also mean gray or blue — all three colors are supposed to be rather pale and calm. A gray horse is “capall glas,” even though a dog or a human is “liath.”)
ruad: red. Specifically a dark red, like old blood. (Dearg/derg is bright red.) Human hair and dog fur is always rua.
Oh, and here’s bonus dog words in Old Irish —
drettel: darling, favorite. Example: “drettli milchu.”
loman: string or rope, but also a dog leash. “cu lomna,” dog on a leash. “milchu ar a morlomain” – a wolfhound on a big leash.
oirce: pet dog, lap dog. “horcae milchu.” Family pets had a special legal position in medieval Irish law, as opposed to dogs that were just owned and used.