Colour indices

A photometric system with more than one band is formally called a multi-colour system (though in practice most photometric systems are multi-colour). For any multi-colour system a series of colour indices, or colloquially colours, can be defined. A colour index is simply the difference between the magnitude of a given object in any two bands. For example, in the UBV system the $B - V$ index is simply the $V$ magnitude subtracted from the $B$ magnitude⁵. Multi-colour photometry is usually published as a single magnitude and a set of colours rather than a set of magnitudes.