Natural dyes for textiles is a current topic in chemistry. Alum (an aluminium salt)-pretreated wool dyed with weld (Reseda luteola L.)—a historically important vegetable source of dye for textiles—leads to yellow colours. In connection with the fact that such colours are of low resistance to light, the effect of different concentrations of aluminium ion on the photo-stability of the dye of weld was studied. The light-fastness of the colour of weld-dyed wool decreased with increasing concentrations of aluminium ion used for pretreating the wool. As the gain in light-fastness by the use of low concentrations of aluminium ion was limited, this cannot be a way to meet today’s requirement of light-fastness of the colours of dyed textiles by itself. Nevertheless, it could be part of a broader strategy to address the need for increased light-fastness of the colour of wool dyed with weld.