Camouflage is the art of hiding an individual or object from plain sight using technological or natural means. It is often used by the military forces of the various intelligent species of the Milky Way Galaxy.
Physical camouflage involves using colour or patterning to conceal or break up the easily recognisable outlines of enemy forces.
Imperium of ManEdit
The camouflage utilised by the Imperium of Man is based on physical paint schemes, many of which are "Codex Approved" schemes that will be utilised by Imperial Guard Regiments and Space Marines Chapters. The schemes depend on the environment in which they will be fighting. As some planets have very different ecologies and geologies and enviroments, some of the colours used in these schemes would seem quite garish to modern eyes. Some Chapters of the Space Marines adopt a no camouflage rule, instead adopting an honorable attitude similar to 18th Century Terran armies of the 2nd Millennium, hoping to intimidate their opposition.
The Orks tend not to use camouflage, with two notable exceptions: the first is that of the Blood Axes clan who imitate the culture of the Imperium but do not understand the theory of camouflage and instead use combinations of colours and patterns with little relation to the environment in which they will be fighting, although it can sometimes prove effective. The second exception is that of Ork Kommandoz, those Ork Boyz who are "kunning az Mork", and are adept infiltrators and saboteurs, and use camouflage, misdirection and discreet movement with an aptitude baffling for such brutish creatures.
The Tau use patterns of phyical camouflage as well as flat colours to match ambient environmental backgrounds.
The Necrons are not known to utilise camouflage patterns.
The Tyranids are not known to utilise physical camouflage.
Some races use holographic technology to either completely conceal or disruptive patterns to dazzle or confuse the viewer. Typically the Eldar and Dark Eldar use holographic technology on their vehicles and starships while the Harlequins utilise personal holographic technology. Cameleoline cloaks are used by a number of races to provide camouflauge for their elite infantry.