Why is a red giant red in color?
The colour of a star is related to its surface temperature, red giants are cooler which gives them their reddish colour.
Man sequence stars have high surface temperatures and generally appear white as they radiate at most visible wavelengths.
Red giants are very large but their surface temperate is lower at below 5,000K which makes them look orange to red depending upon temperature.
Actually most red giants are orange in colour. A star needs to have a surface temperature < 4,000K to appear red.
The reason for the lower surface temperature is that the star is very large and the fusion reactions are concentrated around the core.
The Sun with a surface temperature of 5778K is actually white in colour. It appears yellow due to the scattering of the blue light component by the Earth's atmosphere.