Mostly whatever is clean.

I like Filson's gray, and tie dye shirts in woods shades. I don't buy the commercial stuff any more. Most of it is so photographic I think it mostly sells on the rack, rather than breaking up the outline. I think the texture of the material is at least as important as the colouring.

Come to think of it, there was a show on the Army sniper course recently, where one of the guys during a stalking test was discovered because he hadn't broken up the pattern on his camo glove. Even though camo, it blobbed out pretty badly. That is a pretty extreme case though.

By the way, what is the current line of thought on fluorecense? There used to be a product that was supposed to cut the whitener glare that a lot of cloathing carries. Don't hear anything about that any more. Is that still an issue.