For historical reasons, the Optical & Infrared wavelength range is often divided into several broad photometric bands. Specific filters are used to isolate the emission within these bands, which are label using a letter eg U-band, B-band, V-band...etc. (although the "-band" part is often dropped).

[fromM.V.Zombeck, "Handbook of Space Astronomy & Astrophysics"]

The fluxes are usually quoted in magnitudes (see above), and the differences between the magnitude seen between two bands is referred to as the color

eg B - V color meaning the difference (in magnitudes) between the B-band and V-band fluxes.