Integrated U − J (top) and B − K (bottom) colour as a function of the sSFR of model stellar populations with different SFHs: log-normal (blue circles), exponentially declining (red stars), delayed exponential (black crosses), and inflating SFH (cyan asterisks). There is a close correspondence between the integrated colour and the sSFR, which is largely independent of the SFH law. Solar metallicity BC03 SSP models were used to compute the colours.