The problem with something that murders from nowhere is just that, it murders from nowhere. You don't know what to fear, just... something. Out of sight, out of mind, so to speak. Highly visible shock troops do just that, shock. They are a visceral, visible reminder of power and authority. There's plenty of value to both tactics, but saying that one is scarier than the other is just ignoring the valid uses of both tactics. While I fully believe that Space Marines, even ones like the Ultramarines, would use camo when the situation called for it. But also, they exist as a visible manifestation of the Emperor's will and the authority of the Imperium. In a lot of ways, they want to be seen. As players of a game with codified, "balanced" rules, we're not afraid of Space Marines. In reality, they are seven to seven and a half foot tall, half ton, heavily armed, nearly invulnerable, totally implacable and remorseless killing machines. If Space Marines have been deployed, and you're a rebel, it means your death is near, and somebody has gotten tired of negotiating with you.
The Space Marines certainly employ both tactics. But there's a very specific reason they'd be highly visible. After all, in the kind of battle they engage in, you're not trying to be obscured. Survivability comes from mobility, maneuver, combined arms, and decisive action. It's the reason very few military forces bother to field an urban camouflage pattern. You're never going to look like a building.