First, there are no such things because such objects form very hot and take literally trillions of years to cool anywhere near absolute zero. Only at absolute zero would a body not emit ANY radiation. Now, if you mean optical light, the neutron star would have to cool off until its temperature fell below, say, 1000 degrees. It would appear black since it emits no optical radiation. What would happen if you shined a flashlight on it, or tried to look at it with reflected light from a companion star? Although the crust is pure 'neutronium', its outer surface is a crystalline crust, but that doesn't mean it would be transparent like a diamond. It still has a density of near- nuclear densities, and it may still have a very weak plasma of trapped electrons and protons moving around. As you know, the more conductive a surface is, the better it reflects light. Even glass is transparent, but because its surface atoms are photo reactive, a bare glass surface reflects a few percent of the light that falls upon it. I do not know exactly how conductive the surface of a neutron star might be, but it would not surprise me to hear that the surface of a neutron star might have, at optical wavelengths, rather high reflectivity ad a dull silvery hue. It would be totally black and non-reflective, if the crust were completely non-conductive with no electrons capable of interacting with light at energies of a few electron volts. Personally, I think the latter is more likely and would vote in favor of a dark grey or black surface with a reflectivity of less than 1 percent.