I have all the colours, and I find both the black and grey to be stain resistant. I like to change it up from time to time and after a couple of uses of the cyan and yellow it gets dirty - just wash it and its fine.
Cyan has some kind of food/oil stain that won't wash out. It's not really noticeable from a distance though.
Grey is relatively new but it doesn't have any stains at the moment.
Black is pretty much spotless and seems to have a different texture than the other colours (looks shiny and smooth). I honestly think black looks the best since when it's on the phone you can't tell that there is a case.
To wash them I just take them off the phone and scrub them with a drop of dish soap and a scouring pad, then rinse with water. I also wipe my phone every now and then with a little rubbing alcohol and a paper towel.