Why are molecules coloured?

Generally, molecules are coloured as they absorb visible frequencies of light, to promote an electron to a higher energy level, so the other (complementary) frequencies of light pass through the sample and are seen. However normally these energy levels are too far apart to correspond to frequencies of visible light.(change in E = hv) In organic molecules, a system of alternating single and double bonds (called a conjugate system or chromophore) usually made of benzene rings or c=c bonds, causes electron energy levels to get smaller and closer together. In transition metals, the metal centres are surrounded by ligands, which causes the normally equal in energy d orbitals to split into two sets of orbitals with different energy levels, which are close enough together in energy that electrons will absorb visible light photons to be promoted from the lower energy d orbitals to the higher energy d orbitals.