Skin markings are pretty complicated for an animal. Each cell somehow needs to "know" what color it should be, potentially different from its neighbors, without carrying any information unique to that cell.

Imagine billions of LEDs, and you want to make them display a picture, but you separately program them, you can't address them individually, and you can't even really have them talk to each other. You can sort of flood them with broadcasts of different strengths to influence how they turn on. The sorts of patterns you could form would be fairly limited.