colors colours have special meaning to the Greenskins (Orcs & Goblins and Orks) of Warhammer Fantasy and Warhammer 40000. It has origins in the original comedic elements of Warhammer, with the greenskins heavily influenced by soccer football hooligans during the 70's-90's when rowdiness and fanatical obsession led to frequent riots and embarrassing mania; sports teams and fans are generally superstitious.
In Fantasy this works through magic and actually functions in a way that other races (or at least Wizards) can observe and understand. In 40k this occurs because all Orks share a psychic link allowing them to impose belief on reality (making an engine and cockpit made of random metal welded together and partially held together with sinew and feces into a functional aircraft for example). Thus for them, simply painting something is the same as engineered or enchanted upgrades for other races. Body paint or a skin mutation is likewise an amazing thing. Sky color, water color, any color has meaning.
Belief in Colorz is instinctive as Greenskins are simply an apelike race that reproduce by dying and releasing zygote spores like a fungus, who are "born" with all the knowledge they mostly need including speech and belief in their various gods. The old Fantasy continuity when Orcs had females and young as well as could breed with humans predates Colorz.
Objects or beings that are red go faster. Red can also be described as brutal since going faster helps get into melee. The most well-known and memetic color, because "Red goes fasta." Believed to have started as a reference to the first "Gundam" anime of all things. Red is also associated with relatively disciplined and tactical Orks. It is the color of Kommandos and the Blood Axes clan, who both use advanced tactics such as ambushes, unit organization, and cover.
Blue things are lucky. This goes both for safety and gain. Blue is also cunning, and in Fantasy represents magic itself as well as the physical protection that magic warpaint brings to Savage Orcs. The Gnoblar army Gnoblar Horde: The Unwashed Masses used it to differentiate an entire type of soldier separate from other Gnoblar, the Lucky Gits.
Existing only in Gnoblar lore, Blood-Gnoblars are rangers and trackers that are far smarter than other Greenskins, at least in terms of survival skills and a disturbing amount of common sense. Originally they were a subrace of Greenskin with red and brown skin, but common green Gnoblar wanting to join them began killing things and painting themselves in blood.
The best, because it is their own natural skin color. Anything can be improved with green, even if other colors actually add powers. Fantasy greenskins may paint their non-greenskin allies like Giants and Ogres green to help them be Orcy while Orks paint their human allies, the Diggaz, green as well. Any attempt at diplomacy should involve the color green.
Wealth, although not because of gold; Orc/ks use Teef (teeth) as currency, and their teeth yellow fast. Orks also believe yellow produces better explosions, while Orcs do have a slight value for precious metals although they mostly just pile it up as a luck totem to bring better loot and Teef (it works).
Death, like bones. White is dangerous, and can have multiple meanings although generally it means fearlessness and intent to kill.
Toughness. Black Orcs get their name as a description (that and the fact their metal armor tends to be tarnished and blackened) rather than the color their bodies are due to this.
Regardless if it actually matches the environment, greenskins who think they have camo on will actually seem near invisible. Likewise they may not register camo that they see.
Checkerboard patterns, usually white and black, can mean a connection to Gork and Mork (or was it Mork and Gork?), increased physical prowess, or is a means of showing allegiance; in 40k checker patterns mark an Ork as belonging to the Goff Klan (clan) but in Fantasy the meaning is less clear. The Beast Arises series had a group of Orks infiltrating a Black Templar ship having Luna Wolves Black and White armor, later on a human hypothesizes that the Orks remember the Luna Wolves conquering Ullanor and regard the Luna Wolves' colors as a sign of toughness. It could also be that the Orks had the belief before the Luna Wolves came, but that the armor matched their kulture's belief. A popular meme is that it is Orc/k camo attempts with white being daytime camo and black as nighttime camo.
A purely meme color. At one point the community noticed no Games Workshop paintjob had yet featured purple, and fans began to joke that its because purple is for the greenskins stealthier than using camo, and more cunning than those using blue since nobody had ever seen a purple Orc/k. It has been repeated as if fully canon for long enough GW itself has taken notice; in interviews writers have mentioned it, and several mentions have been made as jokes in Black Library works. Duncan Rhodes brought it up as a camouflage color for Orks in a recent video which is a great wave to the fans.
Another meme color, referencing the paint job that Rick Priestley, creator of both Warhammer games, used to paint his Orcs in an old Warhammer Fantasy Army Book paint section. It has no canon meaning, just being his preference to base coat in red and paint grey skin on top while leaving red visible in the cracks, but some in the fan community have suggested that the original Orcs and Goblins which reproduced sexually and could be found in the same warbands as humans are the greyskins.