Why are some stars red and others blue?

Answer: Surface temperature sets the color

Different chemical compositions — Wrong. Composition changes which dark lines appear in a star's spectrum, but the overall color comes from how hot the surface is, not from what the star is made of.

Age determines star color — Wrong. A star's color can change as it ages, but that happens because its surface temperature changes. At any given moment, color tells you the current temperature, not the age.

Surface temperature sets the color ✓ — Correct! A star glows like a hot object, so its color tracks its surface temperature. The hottest stars shine blue-white, like Rigel and Spica, with surfaces above 30,000 K. Medium stars look white or yellow, like the Sun at about 5,800 K and Sirius. The coolest stars glow red, like Betelgeuse and Antares, around 2,000 to 3,500 K. The peak wavelength shifts toward blue as temperature rises, which is why the O-B-A-F-G-K-M sequence runs from hot blue stars to cool red ones.