The insides of ALL optical devices (cameras, binoculars, telescopes, etc.) are painted flat black to absorb and de-scatter stray light, which includes long wavelength infrared where the heat is.
The protocol for ALL optical and thermal sensing devices used at USArmy Yuma Proving Ground is shiny WHITE/top surfaces with flat BLACK/bottom surfaces.
That's how my solar shields are now, however, there IS a caveat, not all white paints are 100% reflective and not all black paints are 100% absorptive...AND, some paints want to dissolve/eat the plastic plates (depends on the plastic). For now, I'm using Rustoleum™ Flat Black and Gloss White...but I'm interested to hear what others use.