Explanation and physical joke:

I once told a girl about the stars. It was great because she didn’t understand anything. Until I asked if there were green stars.

It seems that by temperature the spectrum should pass through all colors, but in stars for some reason only the main sequence - brown, red, yellow, huge red, blue, white, neutron, quartz, black hole.

> I have been thinking for years. Probably, there are no elements on the surface emitting in the green spectrum. On top, hydrogen and helium are mostly green, I don’t even remember what can radiate.

The narrator apparently did not understand the girl anymore. Neutron stars and black holes in no way belong to the main sequence, they are the final stages of the evolution of stars. The star's radiation spectrum, except for small particles, is a heated black body spectrum and does not depend on the star's chemical composition. And the spectrum of that black body is a smooth curve: in cold stars most of the energy falls on "red", in hot ones - on "blue", and in something like our Sun - just on "green" (but in the sum we perceive its color as white).

The promised joke:

Two physicists meet in the winter. One to another and says:
You have a nose! It is 720 nanometers.
So so small?
and no. such red