Cavy colour genetics are fairly easy to learn if you can memorize some letters and what they represent.

For cavy genetics we use upper case letters to represent dominant and lower case to represent recessive. Recessive genes are usually masked by the dominant genes. I will list the genes in the dominance order for less confusion.

Phenotype is what the animal looks like (the genes you know it must have to show those colours), genotype is what genes it actually has.

Homozygous means to have the same allele for one gene. Heterozygous means to have different alleles for the same gene.

We use the letter S for the white spotting gene S = no white spots, s = white spots. I believe that there is incomplete dominance and pigs with Ss (carries white spotting) may have stray white hairs or white toes. This being said even an ss cavy can have very little white on them.

The extension genes can be very confusing. E = full extension of black pigment. ep = partial extension, this creates patches of colour like you would see on a tortoiseshell (black and red). e = full restriction of black pigment creating a red pig.

A = agouti, hairs with a base colour and a tip colour. The belly shows more tip colour creating the illusion of a solid colour stomach (this is called a belly band)
Ar = solid agouti, there is no belly band. The hairs on the stomach are ticked the same as the back.
At - is tan, I understand a tan to look like a self with a belly band and eye circles.
a = no ticking

B = black or lilac colour
b = chocolate or beige colour

P = dark eyes
p = pink eyes, pink eye gene "dilutes" blacks to lilac and chocolate to beige. Mildly affects red creating more orange.

C = Deep rich colours
ck = dilutes red to a dark buff
cd = dilutes red to light buff
cr = ruby eye, turns red to white
ca = himilayan gene. Restricts all red pigment. Black, lilac, chocolate and beige are restricted to just the extremeties. Turns eyes pink 100% of the time regardless of the eye gene.

Rn = roaning of a self colour with white. Produces a solid coloured head and feet.
rn = no roaning