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An object's color index is its difference in magnitude as seen through two different filters, such as B versus V, or U versus B, typically with the bands standardized by the UBV photometric system. The most commonly-cited color index, B-V, is a star's magnitude as observed through the B filter minus its magnitude through the V filter. The color index U-B is also commonly noted. A black body's color index is related to its temperature, and a color index of a star (which is somewhat like a black body) yields a temperature-determination termed a color temperature of the star. Color temperatures are reasonable rough estimates of a star's effective temperature and surface temperature, useful because they can be efficiently determined for large numbers of stars through photometry.
A color index is analogous to a human's perception of color (thus the name) in the following manner: the wavelengths perceived as "blue" are also present when the colors "white" or "gray" are perceived. We perceive the color blue when the ratio of the intensity in the blue wavelengths to the others (red, green, etc.) is high. Like this, color indexes, being differences in magnitude, express ratios of the intensity of different wavelength-ranges.
The color index reflects properties of the observed star, being generally unchanged by distance, making it a primary basis for analysis for stars too far for spectroscopy. However, the observed color index can be affected by properties of intervening matter, such as reddening. This effect is quantified by the color excess (EB-V), which is a star's actual (intrinsic or normal) color index minus its observed color index.
EB-V = (B-V)Observed - (B-V)Intrinsic
A good estimate of the color excess increases the accuracy and value of a star's color index.