Honestly the colour is not important as we're not talking serious radiative loss/gain here. The heat shield works as a thermal barrier for conductive transfer (heat moving through a material due to potential difference).

Convection can be ignored unless you force cool the outside of the heatshield which isn't a bad idea actually (run a flexi tube up from the lower front grill maybe)

The best material would be plastic and as said, covering the outside in tinfoil certainly wouldn't hurt, but I don't think you see any tangible gain.

If you wanted to go super OTT then I'd say make a sandwhich structure out of a porous honeycomb and force feed air through the sandwhich - this way you reduce your conductive transfer though use of polymer and paper materials and reduce the bulk of material to act as a thermal mass and you can actively cool the bulk using forced convective losses...... hurrah!

Oh - Peltiers might be an option as an electronic intercooler too?

If you