The "box shaped" OPFOR ammo crates change colour depending on the player's distance from them. The long, thin ones ("special weapons" and "basic weapons") are unaffected.
At close range, they are a pale yellow-green, but at longer distances they appear the same grey colour as the equivalent BLUFOR crates.
(From the correct distance, the problem can also be reproduced by zooming.)
Screenshots of the same scene, BLUFOR crates on right, OPFOR crates on left,
unzoomed: http://steamcommunity.com/sharedfiles/filedetails/?id=131388177
and zoomed: http://steamcommunity.com/sharedfiles/filedetails/?id=131388225
(Are Steam Community screenshots acceptable in bug reports? They make it much easier to report bugs directly in Arma using the Steam overlay.)
Arma 3 Alpha 0.5.102571