Which of the following metal ions is colourless?

The ions ${\mathrm{Ti}}^{3+}, {\mathrm{V}}^{4+}, {\mathrm{Cr}}^{4+}$ are paramagnetic therefore, exhibit colours due to $\mathrm{d}-\mathrm{d}$ transitions.

The ${\mathrm{Sc}}^{3+}$ ion is diamagnetic with electronic configuration of argon and does not exhibit colour.