Sandage & Walker have shown that, at the same B-V, the stars in the subgiant branch of the globular cluster M92 differ systematically in U-B from the stars in the asymptotic branch. They found that the difference was larger than that predicted by continuum fluxes calculated from model atmospheres allowing for the difference in gravity between the branches. A number of synthetic stellar spectra have been computed to determine the effect of line absorption upon the colours of these low-metal abundance stars. It is found that, whilst the slope of the colour-colour curve is altered by the inclusion of line absorption, the amount of such absorption is too small to explain the observed difference in U-B in the original observations. The subgiant branch stars may be divided into two groups and those with the greater ultra-violet excess are also intrinsically the fainter stars of the branch. The models do satisfactorily predict the colours of both the intrinsically brighter subgiant stars and all the asymptotic branch stars. Uncertainty in the transformation of photographically measured colours to the U, B, V system may have caused the B-V colours of the fainter group of subgiant stars to be measured to be redder than the stars actually are. If the B-V colours of this group of stars are reduced by magnitudes it is possible for the models to reproduce the then observed slope of the colour-colour curve and to predict the difference in U-B between the subgiant and asymptotic branches.