The Red Paint Job is an Ork vehicle upgrade. Most cultures in the galaxy place special meaning on the colours they can perceive. Many vehicles in the Ork armoury are painted red as a result of the superstition that it makes them "go fasta!". Because of the cumulative psychic effect of the Waaagh!, this thought manifests itself as a real boost to the operation of a vehicle.
The effect is not limited to vehicles as red is found on just about any item an Ork wants to make stronger, faster, shoot better, etc.
The most fanatical Orks belong to the Kult of Speed and use red liberally.