Well, the reason Chesnaught was pale is because Chespin's line was based on a knight, a hedgehog, the life cycle of a chestnut.
Chespin - little hedgehog with green spiky chestnut surrounding. If you look at its back, you see a small circle
Quilladin - bigger hedgehog. The circle at the back of Chespin's head is where its face would go, as it had more armor on that side. The chestnut armor also expanded, which is what actual chestnuts do when they're almost ripe.
Chesnaught - huge hedgehog. The unripened or premature chestnut itself is brown and the outer covering is green, which is why Chespin and Quilladin are their respective colors. Chesnaught's color came from it being fully grown and after a chestnut is fully grown and ripened, it splits in two and has the pale color.