The optical properties of thin sputter-deposited AlN films on Si were measured. It is shown that they depend on the exact preparation conditions. In the visible range, the refractive index n of AlN is fitted by n = c1 exp(-lambda/c2) + c3, where c1 = 2.09 and c2 = 131.9 nm are constant and c3 almost-equal-to 1.8-2.5 depending on the preparation conditions. It is also shown that the chromaticity coordinates of AlN on Si vary quasi-periodically with the product of the refractive index and its thickness.