The colour of CuCr₂O₇ solution in water is green because:

CuSO₄ is blue because Cu²⁺ ions are blue; CuCr₂O₇ is green because Cu²⁺ is blue and ${\mathrm{Cr}}_2{\mathrm{O}}_7^{2-}$ is orange yellow. Thus, a solution of blue and yellow ions appears green.