Color index

In astronomy, color index is a simple numerical expression that determines the color of an object. Most often, the object is being observed through two different filters - such as U and B, or B and V where U is sensitive to the ultraviolet rays, B is sensitive to the blue light, and V is sensitive to the visible light. The difference of the magnitudes with respect to these filters is called the U-B or B-V color index, respectively, and it is usually negative.