Schoeller Coldblack acts on infra red which is invisible to the human eye. It appears black because it absorbs all wavelengths of
visible light but it reflects invisible infra red.
That's what I've read. I don't know that any of my gear is coldblack so can't vouch for its efficacy in the real world.
There are articles in the interwebz which claim 10°F temperature reduction under controlled condition - exposed to a heat lamp and measured with an IR thermal imaging gun.
Incidentally, and noting @
Biker395's avatar jersey, white consistently tests more effective than Coldblack. I can't seem to keep white clean though.
-Tim-