White markings are epistatic genes. That means they cancel out the expression of any other color gene in that cell. White "masks" every other color, because the ability to deposit melanin in the hair is blocked in those cells. So it affects both solid and brindle markings. It's like painting white paint over the color of the coat.

True chimera is two different embryos (fraternal twins) fused into one. So you have patches of color that are from twin A and patches from twin B. The patches are dependent on which embryo the cells came from. If one is, say, black with white the patches that came from its, for example, chestnut twin can be mixed in with either the black or the white. If twin B had white, also, you can get some of the white patches being part of the chimera.

One interesting thing about chimera horses, color-wise, is that the main coat color may not be inherited in the offspring. Whichever twin donated the DNA for the reproductive cells will pass on its color to the foals. So you could have a black with white markings with one small patch of chestnut, but still get foals from the chestnut horse's DNA.