If you’re harvesting the energy from photons into heat, you’d want black. Any photon hitting your sail would be absorbed, and give you its momentum and energy.

If you’re harvesting the energy from photons into heat, you’d want black. Any photon hitting your sail would be absorbed, and give you its momentum and energy. If you’re harvesting the momentum from a photon into momentum for your spaceship, you’re better off with white. A photon being reflected gives you its momentum twice. It’s first (briefly) caught like the black sail, imparting its momentum, but then it’s re emitted and hurled away, like a cannon, giving even more reactive momentum to the sail