The Transition Elements: Explanation of color of compounds. The colours of the major ions

When transition metals form compounds, the d-orbitals of the metal interact with the electron cloud of the ligands in a way that the d-orbitals do not all have the same energy. The way they are split depends on the geometry of the complex. When the d-orbital is not completely filled, it is possible to promote an electron from a lower energy d-orbital to a higher energy d-orbital by absorbing a photon of electromagnetic radiation with the right amount of energy. The right amount of energy usually happens to be within the visible region of the electromagnetic spectrum. So, the light that isn't absorbed is what we see as the color of the compound.

Name	Formula	Color	Picture
Copper(II) sulfate	CuSO4	White
Copper(II) sulfate pentahydrate	CuSO4 · 5H2O	Blue
Copper(II) benzoate	C14H10CuO4	Blue
Cobalt(II) chloride	CoCl2	Deep blue
Cobalt(II) chloride hexahydrate	CoCl2 · 6H2O	Deep magenta
Manganese(II) chloridetetrahydrate	MnCl2 · 4H2O	Pink
Copper(II) chloride dihydrate	CuCl2 · 2H2O	Blue-green
Nickel(II) chloride hexahydrate	NiCl2 · 6H2O	Green
Lead(II) iodide	PbI2	Yellow

http://www.wou.edu/las/physci/ch462/tmcolors.htm
http://en.wikipedia.org/wiki/Color_of_chemicals