Leaf stems.... red on top, green on bottom

So I'm wondering if anybody has any definitive evidence as to why? The consensus seems to be from light, and it would make sense, considering the light coming from above, but nobody shows any evidence to the mechanism behind it. Anybody got anything?

Often times it's genetic, a phenotype. It can also, rarely, be caused by a deficiency, BUT, if the plant is otherwise healthy then that's not an issue.