A way to determine dinosaurs' colors

When I was a boy, I spent many an hour wondering what dinosaurs actually looked like. This is amazing! I'm wondering, though, how sound it is to extrapolate from what the shape of the melanin container says about colour today to what it said about colour in the past?

I'm going to guess that dinosaurs changed colors according to their environments (green in forests, brown/yellow in deserts). They evolved over millions of years, while skin/coat/scale color tends to change over the span of a couple millennia, at least in modern animals. So there were probably a variety of different colors even within species.

Still, it would be really cool to know for sure.