The colors of any object that is warmer than its surroundings are the most important indication of that object’s temperature. In a star, for example, stars that are bluer in (U-B) and (B-V) are almost always hotter at their surface than stars that are redder in those colors. These colors help astronomers determine the spectral types of stars: “O” stars have the hottest photospheres, “B” stars are the next hottest, then “A,” “F,” “G,” and “K” stars; “M” stars are the coolest.

For objects like galaxies and star clusters, which are collections of large numbers of stars, measuring their colors can help astronomers determine how much of the light is coming from what kinds of stars. If the colors of a distant galaxy are bluer, then it is likely that a higher percentage of stars in that galaxy are relatively hot; and if the colors are redder, then there is likely a lower percentage.