I know about red being a leadership/power association colour, it's not that strange. In case of warhammer 40k though... long story short, orks there are basically a race of stupid creatures that bend reality by "believing in something strongly enough". If they believe that painting a vehicle red makes it faster - it will.
It's not just an "association" or "symbolism", it's literally reality-bending sorcery. They even have super advanced technology that is absurd on the level of "put a lid on that trash can, and now you have a spaceship", and it still works as they intended

Their power also become more potent the more orks present. So... if something doesn't work - it just means that there is not enough orks in vicinity that believes that it SHOULD work.
Anyway... red makes you faster - confirmed