It is the relative proportion of these different colours which give the star its appearance. Red stars emit primarily red light. At still higher temperatures, the stars emit significant proportions of green and blue light, such that the relative amounts of red, yellow, green and blue light corresponds to that of white light. These stars appear white. At the highest temperatures the relative proportion of blue light increases further, beyond that appropriate to white light.
Blue light dominates and the stars are blue.
Wiens's Law For any given temperature there is a unique wavelength at which the intensity of the light is a maximum. If this wavelength is measured, then it is possible to deduce the surface temperature of the star using Wien's Law for blackbody radiation.