Colour arises from d-d transitions: electrons in t2g absorb light of energy equal to Delta and jump to eg. The absorbed wavelength is removed from white light; the complementary colour is observed. [Ti(H2O)6]3+ (d1) absorbs at ~500 nm (green-yellow), appears purple. Factors affecting colour: (1) Nature of ligand (changes Delta, hence absorption wavelength). [Co(H2O)6]2+ is pink; [CoCl4]2- is blue. (2) Oxidation state of metal. (3) Geometry — same metal can show different colours in octahedral vs tetrahedral. Colourless complexes: d0 (no electron to transition) and d10 (no vacancy to transition into).