What is the meaning behind different robe colors? I have seen the same monk in two different color robes and am wondering why. They were dark red and of course orange. Ihave also seen brown, but that was zen, and if any other colors have relevance i would like to hear about them as well. I practice Theravada if that helps with referencing or relevance issues.
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.