My GM has already ruled on this, but I have to satisfy my curiosity. Who would allow a Cameleoline Cloak requisition from the Dark Heresy Rulebook. Basically, it blends you in with your surroundings, making you less visible. Do you see any reason why this would not be compatible with power armor?

The properties of a Cameleoline Cloak can be added to any armor. Details can be found in the Radical Handbook I believe.

I doubt many SM chapters, other than Alpha Legion, would approve the use of such a coating on power armor but I do believe scouts use the cloaks or are at least modeled with them from time to time.

I am not sure I would allow the cloak to stack with the stealth bonus the scout armor gives.

As for the cloak with the power armor, I wouldn't be opposed to it if a player asked.

On the Tabletop, Cameleoline Cloaks are available to scout marines, all of which wear Scout armor. It does grant them a benefit (better cover save). Though not technically available to Space Marines, they may very well be available... they just don't overcome the innate non-stealthiness of being a gorilla in a metal tuxedo enough to let SM in PA be considered "stealthy".

As a DM, I would allow it with proper spent requisition (No permanent application of Cameleoline on Power Armor, though, unless we're talking permanent requisition ala Signature Wargear or such), and I would even allow it to stack with Scout Armor bonuses. Something needs to make a veteran Space Marine scout make a mere human guerilla commando warrior look like a braying donkey in comparison to the glorious Speeeeeeese Maaaaaahreeeeeeeeeeens.

One of the ideas I have been kicking around to help with "stealthy-ness" is to allow characters to get mastercrafted armour but instead of the AP bonus it will the penalty to move silent from -30% to -10%.

In my game, I ruled that the Scout Sergeant of the group could requisition a cloak to go with his Scout Armor. I allowed the bonuses to stack, as they should. Otherwise, there would be no point to seeing the Space Marine Scout models with the cloaks and Scout Armor.

Marines like bold and colorful armor- the Blood Angels don't exactly blend in, and to cover up the armor might disgrace the machine spirit (see the section in DW detailing when they repaint their armor black except for the pauldron, which they leave untouched to appease the machine spirit). I'd definitely allow it for missions where you're dressed in scout armor though.

If you're looking for a stealth ability in standard PA, why not try the Masking Screen from page 165: +30 to conceal, imposes a perception penalty at distance however while the field is on (-10, -20 at 30+ yards I think). It's perfect for ambushes, not as great for shooting at distance.