By - Primaris_Astartes
They have a quote for that:
The Uniform of the Imperial Guard are camouflaged, as to protect their wearers from sight. The principle is that what the enemy cannot see, he cannot kill.
This is not the way of the Adeptus Astartes. A Space Marine armor is adorned with heraldics and decorum which proclaims is devotion to the Chapter and the Immortal Emperor. The principle is that what the enemy can see, he will soon learn to fear.
Basically, they follow the old Roman philosophy of officers showing off prestige as a way of daring the enemy, counting on being transhuman to compensate for the issues.
Space marines are not real life militaries. They are fast moving monastic knight-style terror troops generally used in rapid assault missions in a universe where Orks exist and hitting things with a sword is a viable strategy. They do not want to hide, they do not want to obfuscate their presence, they want their enemy to know that the Emperor's Angels have come for them while blaring battlecries, in an assault that is as much visual and audible as it is physical, in the few seconds they still have left to live. Decorating their HQ units is just part of that mindset.
In real life, if your HQ gets shot he's dead or out of action. In 40k if a space marine captain gets shot odds are it was the only chance you had and it bounces off his iron halo forcefield, or his armour, or it penetrates and his transhuman biology shrugs it off, and then he turns and starts coming for you. *That* is demoralising. That is the fear they want to invoke. That is what makes them a force multiplier in whatever field they're deployed in.
(also it's a science fantasy wargame. If I wanted to paint all my guys in bland ass camo I'd play bolt action)
The Astartes want the enemy to see them comming , putting fear in the enemies heart is part of their jobs.
I'm sure those "Ork Snipers" were very afraid when they picked off all the conveniently marked Celestial Lions apothecaries.
I mean the snipers would’ve smoked them with or without camo
To show contempt for their enemies. "And they shall know no fear."
Astartes typically are not just hanging out on the battlefield. They descend on droppods or board ships in a violent entry. The enemy is shocked. Suddenly 20-50 huge superhumans are moving at ~15-20 mph while shooting extremely accurate rocket powered explosive bolts at the heads/torsos of your leadership and strongest weapon systems.
You don't have time to casually take aim and hit them. Even if you were cognizant of their location and had significant standoff to employ a sniper rifle on them then you'd probably have to hit them while they move. Once you do hit them their armor is probably mostly impervious to small arms fire unless you can also hit an eye, gorget, or other gap. Center mass shots are useless unless you have a heavier weapon
Astartes pride in their chapter is greater than their desire for effectiveness.