The colour of transition metal ions is generally attributed to the presence of unpaired $d$-electrons, which undergo $d-d$ transitions by absorbing light from the visible region.

• $\text{Cr}^{3+}$ has the electronic configuration $[\text{Ar}] 3d^3$. Because it has 3 unpaired $d$-electrons, it undergoes $d-d$ transitions and forms a coloured aqueous solution (typically violet or green).
• $\text{Cu}^+$ has the electronic configuration $[\text{Ar}] 3d^{10}$. Since it has a completely filled $d$-orbital and no unpaired electrons, it is colourless.
• $\text{Na}^+$ has a completely filled noble gas core configuration $[\text{Ne}]$. Having no unpaired electrons, it is also colourless.