The colour of ${\mathrm{Cu}}^{+}$ compounds is

The colour of a compound of a transition element is due to the electronic promotion within the 3d orbitals of the transition element. The configuration of ${\mathrm{Cu}}^{+}$ is $(3\mathrm{d}{)}^{10}$ and thus all the five d orbitals are completely occupied and thus no transition can take place.