To the best of my knowledge, that chart is extrapolated from measured data.The white seems a bit hot. Most non metallic paint has an emissivity around 0.9-0.95, while absorbtivity is highly dependent on colour, whites are down around 0.05, black is 0.9+. A bright white paint should be cooler than the chart indicates. Heat gain is determined by absorbtivity, heat loss by emissivity and convection. That colour curve gif looks like it only factors in convection. Experiment is the best way to decide as paint is highly variable.
From experience (sailplanes in the sun) it's pretty close to reality.
Even in a home shop, having composite structures handle boiling water isn't that hard if it weren't for the foam cores. Swapping Divinycell for Rohacell is a pretty good marketing decision.
Obviously, practical issues quickly numb that idea. My last project could have easily dealt with black wings weren't it for the fuel tanks, which would be boiling off fuel even in our climate