Frankly, I'm highly disappointed that some form of reference to "red makes it bettah" didn't make it into the game...
'da red wunz go fasta!' is a classic Ork reference, and as I recall, it actually DID make it into previous Relick WH40k games, and it simply acts as a nod toward the Ork "fluff material" but it also gives them some more character.
That is in reference to vehicles and what not, each color has it's own meaning - such as blue being lucky, yellow making things explode, and red making things go faster. I don't thing that it was referenced in this game do to the lack of Ork vehicles - the rocket boys where about the extent of it all.
"Green is best"