In transition metals, where does the formation of colour come from?

The answer for this stems from the very definition of what a transition metal is: a metal that can form one or more stable ions with partially filled d-subshell. The partially fill d-subshell part is crucial to this.

When a ligand (species with lone pair) bonds to a metal, the 5 3d subshells all become different energies. This means that energy will be absorbed in order to move the e- from the ground state to the excited state. The resultant light is reflected and missing the frequencies which were absorbed (dE = hf) and gives the complimentary colour to the light which was absorbed. If there is no 3d or it is full then no e- can jump therefor the metal will be either colourless(in solution) or white.