Photosphere

A star's photosphere is a hot outer layer, bright enough to shine, and be visible, at great distances. Stars shine by incandescence, and the star's colour depends on the temperature of its photosphere.

Dim red dwarf stars and very bright red giant stars, have photospheres of similar temperature, making those star's of similar colour. But the dim red dwarf star's photosphere is tiny compared with the red giant star's massive photosphere.

Sol's photosphere is relatively compact, making it hot enough to shine in white-yellow light. Sol is massive enough that, when it exhausts the Hydrogen it has available for Hydrogen fusion, gravitational collapse will cause its core to compact to a heat and density large enough for it to begin Helium fusion. Stars that begin Helium fusion have very dense, hot cores, where their fusion takes place, but their outer layers massively expand. When Sol is in its Helium fusion period, it will have a large photosphere, that it only hot enough to glow in red light.