Ork designs involve heavy amounts of spikes and rivets and otherwise spiky bits (a.k.a. junk) that are stuck onto the object. Chaos designs are also spiky, but have a different pattern. Where Chaos design is based on sadism and inflicting pain on to the enemies, Ork design is based on sloppiness and simple belief that more is always better. So that's the reason they try to stick as much of junk as possible on different things.
Orks believe that the color of their equipment is very important; wherein red is faster, blue is luckier, yellow is shootier, black is hardier, purple is sneakier (have you ever seen a purple Ork? EXACTLY), and green is ORKY, so painting something with all of these colors at once will make an unstoppably fast, stupidly lucky, eardrumburstingly shooty, 'dead 'ard, Lictor-like sneaky, and mean killing machine.