In astronomy, the “color” of an object is quantified as the ratio of the brightness measured in two different bandpasses of light. For example, the ratio of U-band light and B-band light is called “(U-B)” and is a measure of the color of any object. When the ratio of the shorter-wavelength bandpass to the longer-wavelength bandpass is higher, then we say that the object is “bluer” in that color; when the ratio is lower, then we say the object is “redder” in that color.

Usually photometry of astronomical objects like stars and galaxies is obtained in more than one bandpass, and from them numerous colors are determined. It is not unusual, for example, to get a galaxy’s colors in (U-B), (B-V), (V-R), and (R-K), and to combine all that color information to deduce important properties of that galaxy.