Why `Zn^(2+)` salts are white while `Ni^(2+)` salts are coloured?

Why Cd^(2+) salts are white ?

Why Sc^(3+) salts are colourless whereas Cr^(3+) salts are coloured.

Why Sc^(3+) salts are colourless whereas Cr^(3+) salts are coloured.

Given reason : Ti^(4+) salts are colourless where as Cr^(3+) salts are coloured.

Why Ti^(4+) is colourless where as Cr^(3+) salts are coloured.

Why are Cu(I) salts colourless ?

Why do some clouds appear white while some appear grey ?

A white crtystalline salt A reacts with dilute HCl to liberate a suffocating gas B and also forms a yellow precipitate. The gas B turns potassium dichromate acidified with dilute H_(2)SO_(4) to a green coloured solution C.A,B and C are respectively