Paragraph: An aqueous solution of a mixture of two inorganic salts, when treated with dilute HCl, gave a precipitate (P) and a filtrate (Q). The precipitate P was found to dissolve in hot water. The filtrate (Q) remained unchanged when treated with H$_2$S in a dilute mineral acid medium. However, it gave a precipitate (R) with H$_2$S in an ammoniacal medium. The precipitate R gave a coloured solution (S) when treated with H$_2$O$_2$ in an aqueous NaOH medium. Question: The coloured solution S contains