The following ion is colorless in aqueous solution?

$$\begin{gathered} {\mathrm{Ti}}^{2+}=\left[\mathrm{Ar}\right]3{\mathrm{d}}^{2}4{\mathrm{s}}^0\to 2 \mathrm{unpaired} {\mathrm{e}}^{-}\mathrm{s} \to \mathrm{coloured} \\ {\mathrm{Cu}}^{+2}=\left[\mathrm{Ar}\right]3{\mathrm{d}}^{9}4{\mathrm{s}}^0\to 1 \mathrm{unpaired} {\mathrm{e}}^{-}\mathrm{s} \to \mathrm{coloured} \\ {\mathrm{Ni}}^{+2} =\left[\mathrm{Ar}\right]3{\mathrm{d}}^{8}4{\mathrm{s}}^0\to 2 \mathrm{unpaired} {\mathrm{e}}^{-}\mathrm{s} \to \mathrm{coloured} \\ {\mathrm{Zn}}^{+2}=\left[\mathrm{Ar}\right]3{\mathrm{d}}^{10}4{\mathrm{s}}^0\to \mathrm{no} \mathrm{unpaired} {\mathrm{e}}^{-}\mathrm{s} \to \mathrm{colour} \mathrm{less} \end{gathered}$$