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The colour of a star is subjective. The graph below was created by Hertzsprung and Russell. They plotted the spectral type on the horizontal axis, which is the parameter assigned to a star based on its spectrum; which is the way the light appears on a prism or a spectrograph.

The main spectral types are; O, B, A, F, G, K and M, from the hottest to the coolest, the following table contains the general properties of each spectral type.

These 'classes' are then split into another group, the 'spectral type', in more specific detail, such as a star may be a B class, but specifically it will be B2. This gives the temperature of a star, hence the lower the subdivision the hotter the star. The subdivison goes from 0-9, but then they are again split into roman numerals, the 'luminosity class', this refers to the size and its average density. Below is a table of the lumiosity class.

Sometimes a star description of Ia or b will be used, this is to indicate that it is a brighter supergiant, and a Ib is a fainter one.

A star that has a large mass will concentrate its core with a fierce nuclear fire, which produces more energy than a star of a lower mass. Hence a massive main sequence star is bigger, hotter and brighter. This is a fundamental point of stellar astrophysics; mass determines class.