- GL is Just Peachy
Roan means there are fewer melanocytes, so some hairs aren't connected to melanocytes and get no color. Hairs with no color are white.
Lethal whites don't have any melanocytes at all, and that's also connected with nerve and tooth development. Hence their problems.
Every melanosome can make two kinds of color granules, black and red. The agouti allele says "when you try to make black, make a little red first".
Another gene can say, "everyone make black!", "everyone make red!", or "you guys make red and you guys make black". This leads to self black (or agouti), self red, or tortoiseshell (or agouti and red).
If it takes 5 steps to make black, chocolate is step 4. Step 5 is broken. So all the color granules coming out of the "black" color granule side of the melanosomes are unfinished, and chocolate color. Any place there's supposed to be black will be chocolate instead, including in the base of agouti hairs.
Lilac and beige are a problem with how much black (or chocolate) gets put into the hair. The melanosomes just don't spit out nice steady stream of color granules from the "black" side. So you get a very diluted black, which we call lilac. If you have step 5 of black not working, you get chocolate being diluted, which we call beige. The red side of the melanosome is a little bit affected by this, too, so it can't put out quite as much red and you get an orange pig. It's not as big a difference as black->lilac or chocolate->beige. This problem also makes the eyes pink instead of dark.
There's a whole series of alleles (different forms of a gene) that affect the 92802948343 steps of red-making. Some make it more yellow than red, so you get buff or cream anywhere the melanosome is trying to make red. This includes on the tips of agouti hairs.
One screws up the red-making process completely so the red side of the melanosome is spitting out colorless color granules. This means you get white anywhere you should have red--on the tips of agouti hairs (silver agouti), on self red or orange pigs (DEW or PEW), on brindled pigs (magpie).
Another one not only breaks the red side of the melanosome, it makes the black side have a problem in step 2 of black pigment making. If the melanosome is hot, the pigment can't get past step 2, and has no color. If the melanosome is cooler, like on the nose or feet, step 2 works most of the time, and you get however much black pigment, and whatever kind of black, you would normally get. This gives you a himalayan pig in black, chocolate, lilac, beige, silver agouti, depending on other genes.
Spotting is what most pigs with white patches have. It's like telling some of the melanosomes, "Hey, STOP MAKING ANYTHING!! You there, shut up! That group over there, nothing out of you! Everyone else, go about your business."
I hope that made everything clear as mud.