Synthetic spectra, computed for F and G dwarf stars, have been used to compute theoretical UBVand RGU colours. These colours have been used to obtain the transformation equations between the two systems. These equations have been checked against UBV and RGU colours computed for hotter stellar models. The effects of metal abundance and gravity on the theoretical colours of the cooler stars have been examined. It is shown that the location of a star in the U-G, WR diagram is marginally more sensitive to changes in abundance or gravity than is the case for the U-B, B-V diagram. The effect of reddening on the colours is examined.