Why do people in the future always wear black and gray?

Yesterday I saw Terminator: Salvation and I noticed that everyone in this post-apocalyptic world wore only shades of black and gray. It got me thinking that this seems to be the way in all post-apocalyptic future movies. Why aren’t there any colorful clothes in the future? I mean there must be thrift stores with used clothes right? So why don’t some of these folks pick up something with a little panache?