I agree with MariaMaria--no cotton!! It shows every ounce of sweat and holds all that moisture in.
I have the same problem and absolutely could not live without my moisture-wicking workout pants that are the exact same color wet or dry (black is a must, I don't know why they bother making workout pants in any other color). They could be soaked through with sweat and no one would ever know, except for sweat marks I leave behind after sitting down (note to self: try to never sit down at the gym).
There are lots of brands that make moisture wicking workout clothes that won't show sweat. Look for black workout pants made out of lycra, spandex, and/or nylon. The Champion series that Target carries is pretty good. My
favorites are from Athleta.