Differences between PSF and aperture magnitudes of the tertiary stars in the g_Mr_Mi_Mz_M bands as a function of the colour of stars (gray dots). The black points with error bars represent the average deviation and its uncertainty in bins of colour. Typical SNe colours at maximum light are marked with red dotted vertical lines. The blue curve on the top panel shows the effect of the PSF wavelength dependent correction on synthetic magnitudes obtained with PHOENIX stellar models. The colours of BD+17 4708 are marked by the black vertical lines.