The colour of a star indicates its

From Wien's displacement law, we have

${\mathrm{\lambda }}_{\mathrm{m}}\mathrm{T} = \mathrm{constant}$

where ${\mathrm{\lambda }}_{\mathrm{m}}$ is maximum wavelength, T is absolute temperature. That means, as the temperature of the black body rises, the maximum energy emitted shifts towards shorter wavelength. Hence, star appearing reddish has the lowest temperature and that appearing bluish has the highest temperature.