>>29402364Though he also went back to being pretty orange again for DPP/BW, and always has been in the anime.
Besides becoming yellow again I think the biggest change in X/Y is that his facial features got smaller and his forehead got much bigger. His facial expressions also became identical to that of Pikachu whereas he used to have a bit more of his own personality. Uncurling his tail and not standing up on his feet also made him look more like Pikachu.