There is no $\mathrm{d}-\mathrm{d}$ transition in $\mathrm{C}{\mathrm{u}}^{+}$ because $\mathrm{d}-$ orbital is completely filled, but $\mathrm{C}{\mathrm{u}}_2\mathrm{O}$ is coloured due to charge transfer from oxygen to metal which can undergo an electronic transition into an excited electronic state.