Spectral reflection (glossy) vs diffused (matte or flat) makes a big difference in IR absorption.
Color pigment makes a difference, i.e. it might be gray to visible light, but its white to IR.
I ran several "sun light" tests on my gray and didn't see a significant increase compared to white.
I'll repeat these test this weekend if it gets sunny out.
F16 performance on a Piper Cub budget
LongEZ, 160hp, MT CS Prop, Downdraft cooling, Full retract