GRP/ Composites have a long history in the harsh marine environment.
painting /colouring is commonplace and unlike the aviation environment,stressing a structure well beyond it's design limits is commonplace and rarely results in more than a soaking and a lot of ribbing in the bar, afterwards.
There has to be a "middle path" between the High-Vis requirements and the heat-absorbtion problem.
it is NOT insurmountable-As mentioned by other posters, 2-pack, Isocyanate-based Automotive paints have proven adhesion and durability qualities when applied to Alloys and Composites.
Given the mind-boggling prices that a lot of Certified Aeronautical "kit" seems to command, there will be specialist paint-manufacturers out there who CAN develop a heat AND light-reflective finish which is compatible with Aviation requirements.
The likely stalling-point is the beurocracy dragging it's heels and spending years prevaricating before approval.....OTOH. they are well proven to bring in legislation enforcing undeveloped, unproven and in some cases, demands for which NO solution is yet extant.
(cue "crash" development programs and suspension of unworkable rules ,until reality catches up with the beaurocrat's fantasies.
I would NOT be happy flying in a white Composite.....legal or not, I'd be happier with a lightweight LED strobe with a Li-Poly battery,parked on the glare-shield and radiating SOME warning of my prescence, through the windscreen.