They are color-coded by purpose, but green seems to be the most common. On major highways, the signs giving directions to various places of interest, those denoting exits, and those giving mileages to various cities farther down the road are all green. Rest stops and signs near exits giving information about the gas stations/lodging/food accommodations that can be found there are all blue. Signs pertaining to specific places are also sometimes brown, as you can see above.