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Who needs to stealth their way to the objective?

Astartes can just launch a Drop Pod or have TDA suited Astartes teleported right to the target.

Putting it as delicately as possible:
The "mainline" Astartes(ex: the Battle-Brothers in Power Armour) don't need to hoof it to their targets. They don't need to worry about small arms fire, since they can just walk through it.

They aren't generally deployed in the kind of slugfest that the Imperial Guard are(which is partially why the Guard use camo--that and it gives the Guardsmen a smidge bit of confidence), they're the scalpel intended to be deployed in operations where the sheer terror of knowing the Emperor's Angels of Death are coming after you will be enough to quell most resistance.

Plus, as it's mentioned in the whole reason why the Deathwatch retain a shoulderpad in the colors of their parent Chapter, they don't repaint the armor for each campaign due to not wanting to risk offending the Machine Spirit of the armor.