Multicolour observations, inhomogeneity and evolution

We propose a method of testing source evolution theories that is independen t of the effects of inhomogeneity, and thus complementary to other studies of evolution. It is suitable for large scale sky surveys, and the new gener ation of large telescopes. In an earlier paper it was shown that basic cosm ological observations - luminosity versus redshift, area distance versus re dshift and number counts versus redshift - cannot separate the effects of c osmic inhomogeneity, cosmic evolution and source evolution. We here investi gate multicolour observations, and show that by comparing luminosity versus redshift in two or more colours, constraints can be placed on source evolu tion even if unknown source evolution is present, providing an important te st of evolution theories that is complementary to present methods. However, number counts in different colours versus redshift are not useful in separ ating the effects of source evolution and inhomogeneity.