The colour of transition metal complexes is generally due to $d-d$ transitions, which require the presence of incompletely filled $d$-orbitals. Species with a completely empty $d$-subshell ($d^0$) or a completely filled $d$-subshell ($d^{10}$) cannot undergo $d-d$ transitions and are therefore colourless.

• In $\text{TiF}_6^{2-}$, Titanium is in the $+4$ oxidation state. The electronic configuration of $\text{Ti}^{4+}$ is $[\text{Ar}]3d^0$. Since it has no $d$-electrons, it is colourless.
• In $\text{Cu}_2\text{Cl}_2$, Copper is in the $+1$ oxidation state. The electronic configuration of $\text{Cu}^+$ is $[\text{Ar}]3d^{10}$. Since its $d$-subshell is completely filled, there are no vacant $d$-orbitals for electron transition, making it colourless.
• $\text{Co}^{3+}$ in $\text{CoF}_6^{3-}$ has a $3d^6$ configuration and is coloured.
• $\text{Ni}^{2+}$ in $\text{NiCl}_4^{2-}$ has a $3d^8$ configuration and is coloured.