Two metals X and Y form the salts ${\mathrm{XSO}}_4, {\mathrm{Y}}_2{\mathrm{SO}}_4$, respectively. The solution of salt ${\mathrm{XSO}}_4$ is blue in colour whereas that of ${\mathrm{Y}}_2{\mathrm{SO}}_4$ is colourless. When barium chloride solution is added to ${\mathrm{XSO}}_4$ solution, then a white precipitate Z is formed along with a salt which turns the solution green. And when barium chloride solution is added to ${\mathrm{Y}}_2{\mathrm{SO}}_4$ solution, then the same white precipitate Z is formed along with the colourless common salt solution. What could the metals X and Y be ?