In the first part of this work, the empirical correlation of stellar surface brightness F_{v} as a function of the (I_{c}-K) broadband colour has been investigated by using a sample of stars cooler than the Sun. A bilinear correlation is found to well represent the brightness of G, K and M giant stars. The change in slope occurs at an (I_{c}-K) of ~2.1 or at about the transition from K to M spectral types. The same relationship was also investigated for dwarf stars and found to be distinctly different from that of the giants. Their correlation differs by an average of -0.4 in (I_{c}-K) or a maximum difference in F_{v} ~ -0.1, positioning it below that of the giants', with both trends tending to convergence for the hotter stars in our sample. The flux distribution derived from the F_{v}-(I_{c}-K) relationship for the giant stars, together with that derived from a F_{v}-(V-K) relationship and the blackbody flux distribution is then utilized to compute synthetic light V and colour (V-R)_{c}, (V-I)_{c}, and (V-K) curves of cool spotted stars. We investigate the effects on the amplitudes of the curves by using these F_{v}-colour relations and by assuming the effective gravity of the spots to be lower than the gravity of the unspotted photosphere. We find that the amplitudes produced by using the F_{v}-(I_{c}-K) relationship are larger than those produced by the other two brightness correlations, meaning smaller and/or warmer spots.