Answer is: when the size of a star increases, luminosity of star also increases. The color of a star is decided by the surface temperature of the star and the temperature of a star depends on the size of star. The smaller stars have low temperature and they are usually red (Proxima Centauri - red dwarf star). The massive stars have higher temperature and they are usually blue (for exaple Zeta Puppis - Blue supergiant star).
