I wish I could find the sources from when I was reading about this months ago, it was more about evolution in terms of things that can happen and not ‘random’ mutations, and one of the examples was tigers with orange fur instead of green. It’s not physically impossible to have structural coloring (although the fact there are no green mammals suggests a strong inhibition somewhere along the line), but you first have to have the genetic and molecular groundwork laid to allow it to happen. Ex: it’s not physically impossible for animals to manufacture their own vitamin C, but humans just can’t do it because we don’t have the necessary molecular pathways other animals use. I hope that makes sense for what I’m trying to get at.