The blue supergiant Theta Muscae has one of the lowest B−V indices at −0.41,[4] while the red giant and carbon star R Leporis has one of the largest, at +5.74.
In principle, the temperature of a star can be calculated directly from the B−V index, and there are several formulae to make this connection.
[6] A good approximation can be obtained by considering stars as black bodies, using Ballesteros' formula[7] (also implemented in the PyAstronomy package for Python):[8] Color indices of distant objects are usually affected by interstellar extinction, that is, they are redder than those of closer stars.
M. S. Bessell specified a set of filter transmissions for a flat response detector, thus quantifying the calculation of the color indices.
However, these labels, which have a quantifiable basis, do not reflect how the human eye would perceive the colors of these stars.