Near-infrared and optical imaging of BL Lac host galaxies is used to investigate their colour properties. We find that the R–H colour and colour gradient distributions of the BL Lac hosts are much wider than those for normal ellipticals, and many objects have very blue hosts and/or steep colour gradients. The blue colours are most likely caused by recent star formation. The lack of obvious signs of interaction may, however, require a significant time delay between the interaction event with associated star formation episodes and the onset of the nuclear activity.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html