Most of the compounds of transition metals are coloured in solid as well as solution state. Colour in transition metal ions is usually due to electron transition within the d-shell. Intensely coloured ions with the metal in its highest oxidaton state (e.g., Mn7+, Cr6+,Fe6+) derive the colour from electrons transitions between the metal and the oxygen atom. Iron (i) Fe2+ (ferrous ion) is green. (ii) Fe3+ (ferric ion) is pale yellow in colour. Chromium (i) The colour of chromic ion, i.e., Cr3+ depends around the ligand around the ion. Aqueous solution contains the violet octahedral hexaaquachromium (III) ion, but when some of the water ligands are replaced by other species, such Cr3+ ions are green. [Cr(H2O)6]3+(aq)+3OH−(aq) Violet,octahedral ⟶[Cr(OH)3(H2O)6](s)+3H2O(l) Green, octahedral (ii) In chromate ion (CrO42−),Cr occurs in +6 oxidation state and it is yellow in colour.