For basic colors, I like any kind of blue (royal, navy or sky), white, black, and gray. For louder colors, I like orange (not burnt orange, or gay peach orange, just orange) or green (forest green is fine, though I prefer regular old St. Paddy's green, anything but gay minty green). Red and yellow are tolerable, but probably my least favorite of the "normal" colors.
Please, dude, seriously, really, please, no burgundy, no salmon, no pink, no purple, no fuscia, and no bright neons. Tye-dye is cool if it's the bright multi-colored ones like they wear at Joe's Crab Shack, and you know I won't object to camouflage!
To sum up, since girls look good in most any color, make it something a guy can wear anywhere and with any kind of pants or shorts, like out to the bar or to the Royals game or out to eat, and he won't look like a flaming... well you know.