If enough orks believe it, their combined latent warp powers starts bending reality to make it true.
Example: Red is faster, Yellow is wealthier, Blue is luckier
But it only works because you got billions of ork empowering it, and because the ork do not doubt it(too much).
Warboss? Ork who as killed, killed, killed, killed and killed even more, unilt he got recognized as something more than a Nob. Then the latent energies makes him taller, because "the tallest is the strongest".
The entire reason the orks are not invinicble is because:
*There can easly sneak in some doubt, somewhere, that doubt can spread, ergo they can not be invincible
*It would be boring
If 1 ork belivies something, if the thing is small enough, it might happen over time. If a entire wagh of several billion orks believe something small, it becomes true(red is faster! Guns do not need to reload!), the case with guns means that an induvidual see somebody reloading, and then start doubting, and thus it spreads.