How do you rate coloration?

I just pay close attention to the color as I'm pouring it into a glass. The rate of pour is pretty much the same, whether it's coming from a bottle, a can, or a tap - so you are always looking at a stream of beer that's about the same thickness. That eliminates all the questions about whether you should be looking at the edge of the glass, or the center. Or if you need to be back-lighting it with your phone, or a window, or a mercury vapor dusk-to-dawn light. Watching the color of the pour as it enters the glass provides a consistent, repeatable reference point regardless of the beer style.
That having been said, I think color is one of the less important rating criteria. I don't drink beer because it's pretty.