Coloured illusory triangles due to assimilation.

A coloured addition by Varin to Kanizsa's triangle figure gives rise to a colour-tinted illusory triangle. Two theories for this effect are described, (i) a cognitive theory based on perceived transparency and (ii) a theory based on an assimilation mechanism. An experiment is reported which uses certain colour combinations incompatible with transparency and other combinations where the two theories predict different illusory colours. The results support the assimilation theory.