What physical change causes a star transitioning into a red giant to appear distinctly red?

Answer

The cooling of the expanding outer layers

The immense outward pressure forces the outer envelope to expand dramatically. As these large outer layers move outward, they cool significantly, causing the surface color to shift toward the red end of the spectrum.

What physical change causes a star transitioning into a red giant to appear distinctly red?
astronomystarsstellar evolutionmain-sequencered giant