Question

why are compounds of copper coloured while those of zinc colourless

Answer 1

copper salts are blue green in colour because they absorb light at a wavelength of between 580-630nm. 

zinc salts however do not absorb light at wavelengths between 400-700nm... the human eye is only able to detect wavelength spectrums from between 400 nm to 700 nm and therefore it will appear colourless to us.

Answer 2

Zinc is not a transition metal. while copper is a transition metal it means that it has incomplete d orbit. The lone pair present in a solution causes repulsion. This results in the energy changes in the d- orbital due to which it achieves photon absorption, hence giving the compound its color.