Existing efforts have mostly been focused not on the visual portion of the spectrum, but on IR, since that's where military vehicles tend to stand out most starkly against the background.

Here's some links, which should give you some idea of what the performance is like:

http://defense-update.com/20110512_eltics_black_fox.html

http://defense-update.com/20110905_bae-adaptiv_camouflage.ht...