What is the meaning behind different robe colors?
I believe initially the robes were mostly yellow as this was a color of renunciation in the locality at the Buddha's time. As the Dhamma-Vinaya spread the different robe colors occurred due to the local dyes monastics used in their respective localities being different shades and colors.

I do know that in the Theravada tradition at least in Thailand and Sri Lanka there is a differentiation in colors between forest monks and city monks. 

This may be helpful : http://www.buddhanet.net/e-learning/buddhistworld/robe_txt.htm

The robe dye is allowed to be obtained from six kinds of substances: roots and tubers, plants, bark, leaves, flowers and fruits. They should be boiled in water for a long time to get the dun dye. Saffron and ochre (from the jackfruit's heartwood) are the most prevalent colours today. Though there is a tendency amongst forest monks to wear ochre and city monks to wear saffron, but this is not always the rule.