Here are a couple of sunspots from last week. I've been looking into how they form. They are colder patches formed because concentrations of magnetic flux get in the way of convection currents near the outer surface of the sun. The centre of a sunspot, the umbra, is quoted as being 3000 - 4500 Kelvin whilst the surface of the sun is usually 5780 Kelvin. Normally, anything at the temperature of a sunspot would be glowing very brightly and would be quite a red colour. The luminosity is proportional to the fourth power of the absolute temperature by Stefan's Law. This means the usual surface is say 5780^4/4000^4 = 4.4x brighter. The contrast makes it look dark rather than red.
