The colour of transition metal ions in aqueous solution is generally due to the presence of unpaired $d$-electrons, which can undergo $d-d$ transitions.

• $\text{Sc}^{3+}$ has a $3d^0$ configuration (no $d$-electrons), so it is colourless.
• $\text{Cu}^+$ has a $3d^{10}$ configuration (completely filled $d$-subshell), so it is colourless.
• $\text{Ni}^{2+}$ has a $3d^8$ configuration (2 unpaired electrons), so it is coloured (green).
• $\text{Ti}^{3+}$ has a $3d^1$ configuration (1 unpaired electron), so it is coloured (purple).
• $\text{Co}^{2+}$ has a $3d^7$ configuration (3 unpaired electrons), so it is coloured (pink). Therefore, the pair in which both ions are coloured is $\text{Ni}^{2+}$ and $\text{Ti}^{3+}$.