Copper sulphate and alum crystals. Copper (II) sulphate (blue, CuSO4) is a chemical compound of copper and sulphate ions. The copper is in its +2 (II) oxidation state. The blue colour is due to the formation of a hydrated complex of the copper ion. Potash alum, or potassium alum (white, KAl[SO4]2) is a chemical compound of potassium, aluminium and sulphate ions. Chrome alum, or chromium alum (dark purple, KCr[SO4]2) is a chemical compound of potassium, chromium and sulphate ions. Chromium and copper are transition metal elements, and form coloured compounds, whereas potassium and aluminium usually do not, so potash alum is colourless.