Okay, there are a few things to consider with rabbit colors. All colors are made from the pigment melanin. There are two main options, the dark colors come from eumelanin, the yellow/orange/fawn/red shades are pheomelanin. The dark eumelanin comes in two versions, the dominant black option, and the recessive brown option, called 'chocolate', which are found on the B gene (think 'b' for black & brown). These are the full-strength, or 'dense' colors, which are dominant. The recessive versions are called 'dilute', where the colors are muted or diluted. Black becomes gray (called 'blue'), and chocolate becomes a pinky-beige (called 'lilac') These are on the D gene (think 'd' for dense and dilute). See, you've already got two of the five main color genes accounted for.
Agouti is wild rabbit color. It has two distinctive traits: 1) There are multiple colors on the hairshaft. The agouti gene turns hormones on and off that turn the dark and yellow melanin factories on and off, making bands of color. Generally, the bands are an outer dark band, a middle yellow band, and a base lighter dark band. 2) Agouti also has a pattern involved, with a light belly, light inside the ears, light eye rings, and light under the chin.
Agouti can come in any of the four dark colors: Black agouti is called 'chestnut agouti', or 'castor', depending on breed. The kits are born black, but even at birth you can see the inside of their ear is light. As the hair grows in, you will begin to see the banding on the hair, with black on the tips, chestnut brown (a mixture of black and fawn pigments), then a yellow band, and a slate gray base.
Chocolate agouti is also called 'cinnamon' in some breeds. They are born chocolate brown, with that same light inner ear, and will develop brown tips,with bands of chocolate, fawn, and a pinky-beige base. Blue agouti is called 'opal', the kits are born gray (not black) with the light inner ear, and develop a mostly shades of gray coat, with the white agouti patterning. Lilac agouti is called 'lynx', and is unique in that it has more of a fawn coloring (much of that has to do with the standard of perfection in some breeds that requires a white undercoat, which is related to the fawn genetics.) Again, you'll see that same white patterning of light belly, white eye rings, light inside the ears, and a fawn triangle behind the ears.
So, the first thing I look at when trying to determine color on a newborn kit--is does it have light/white inside the ears? If so, you are looking at an agouti-based color. It is easiest to tell which of the four main color options at birth, because the chestnut agouti will be black with white inside the ears (not all-white ears as with the White Ear gene, that's a whole different ballgame.) The cinnamon will look chocolate brown, the opal will be gray (not black and not brownish), and the lynx will be a light brownish color (not gray and not chocolate brown).
If you have determined it is an agouti, the next question is--do you have chinchilla/ermine in your herd? If so, the yellow pigment factories will be closed down, and so the agouti pattern will have pearly white everywhere the normal pattern will have yellow/fawn shades. The best place to see this will be behind the ears. The normal full-color (C gene) rabbit will have a yellowish triangle of fur behind its ears, on the back of the neck. In chinchillas, that triangle will be pearl white. The rabbits will look the same as full-color agouti rabbits at birth, the differences will become apparent once the banding grows out, and you don't see any yellow.
Agouti has a recessive side, called 'self'. This is where the rabbit is the same color all over. The belly isn't white, it's the same color as the rest of the rabbit. No eye rings, no banding on the fiber. The same four options are available in self-colored rabbits: black, chocolate, blue and lilac. The blacks will be born black with black inner ears and black bellies, the chocolates will be be all chocolate brown, the blues all gray and the lilacs a soft brownish shade.
Tan rabbits are like a hybrid, they have the agouti patterning with the eye rings and light belly, but the hair is solid like a self. This is where you get otters and martens. Otters have the full-color C gene, martens are the chinchilla/sable version. So, Tan is recessive to agouti, but dominant over self colors. You can tell marten/otter kits when the agouti pattern develops but not the rings on the fur.
If the rabbit is yellow (red/orange/fawn/cream), and has the self non-agouti gene, you will see one of the four main dark colors on the face/ears/feet/tail, and yellowish on the main body. We call these tortoise shell, 'tort' for short. It comes in black tort, chocolate tort, lilac tort and blue tort. Again, look at the kit when it is young, it's easiest then to tell black from chocolate brown from gray (blue) and lighter brownish (lilac). If it is yellow and has the agouti gene, it will be red/orange/fawn/cream all over, and have the white eye rings and light belly and light inside the ears Some breeds call this color orange, some fawn. The red rufus modifier will turn the rabbit redder. Dilute will make it lighter. (This is the most recessive color on the 'E' gene, called non-extension, or fawn.)
There are other options, like harlequin or steel, pointed whites, ermine, albino, Vienna blue-eyed white, broken (spotted), sable. . .but this will help with the most basic of colors. Look for the nestbox color to see if you are dealing with a color based on black or brown, and whether it is full dark black or chocolate brown, or a lighter shade (a dilute). Then we just add that color to the pattern.
