Out of ${\text{TiF}}_6^{2-}$, ${\text{CoF}}_6^{3-}$, ${\text{Cu}}_2{\text{Cl}}_2$ and ${\text{NiCl}}_4^{2-}$, the colourless species are:

A species is colourless if it has no unpaired electrons ($d^0$ or $d^{10}$ configuration) to undergo $d-d$ transitions.${\text{TiF}}_6^{2-}:{\text{Ti}}^{4+}\text{ is }3d^0$ (Colourless).${\text{CoF}}_6^{3-}:{\text{Co}}^{3+}\text{ is }3d^6$ (Coloured).${\text{Cu}}_2{\text{Cl}}_2:{\text{Cu}}^{+}\text{ is }3d^{10}$ (Colourless).${\text{NiCl}}_4^{2-}:{\text{Ni}}^{2+}\text{ is }3d^8$ (Coloured).Thus, ${\text{TiF}}_6^{2-}$ and ${\text{Cu}}_2{\text{Cl}}_2$ are colourless.