The difference between the apparent magnitudes of a star at two different wavelengths, as measured through filters of different colours, e.g. B (blue) and V (yellow-green). B − V and U − B are typical colour indices. In Johnson photometry and Kron–Cousins RI photometry, all colour indices are made zero for a star of spectral classification A0V (e.g. Vega). Generally, hotter stars have negative colour indices, and cooler stars positive ones. Nowadays colour index is usually shortened to ‘colour’.