An equation of state for dense nuclear matter such as neutron stars

Some of the oldest neutron stars may have already cooled to ordinary temperatures of just a few hundred Kelvin.

It's not certain what their surface would be like, physics just isn't developed enough to provide a definite answer.

The trivial case of an isolated cold neutron star is: completely black, other than a very slight far infrared glow of whatever their remaining thermal radiation is.

An educated guess is that an externally illuminated cold neutron star would look extremely smooth and "dark grey". Most current models propose that young and hot neutron stars have an atmosphere of metal plasma. Presumably this would freeze out into a thin surface layer as the stars cool.

A complication is that their surface gravity is so high that inbound "white" light would be shifted well into the UV range, which would interact with the surface very differently to visible light. Reflected light would be redshifted back out to the original white, but any light that's absorbed and re-emitted would likely take on interesting colours.

Here's a chart of metal surface reflectivity by wavelength: https://www.researchgate.net/figure/Spectral-reflectivity-of...

As you can see, most metals don't reflect much in the UV range, but there are peaks and troughs.

Depending on the exact surface composition, you might see different colours than what you'd normally expect for metals.

You'd also have to take the rotation rate into account. If there's any inconsistency in the surface albedo, you'd see "horizontal stripes" for fast-spinning stars, or flickering for slower spinning ones.

It would probably shine like a blackbody based on whatever temperature it is.

The surface of a neutron star is covered with a thin shell of protons and electrons in essentially random configurations, so it would absorb and emit light in all sort of random frequencies.