It is observed empirically that $\mathrm{Ln}{}^{3+}$ ions with $4f^n$ and $4f^{(14-n)}$ configurations have similar colours. Which pair of ions is expected to have similar colours?

1. A$\mathrm{Ce}{}^{3+}$ ($Z=58$) and $\mathrm{Eu}{}^{3+}$ ($Z=63$)
2. B$\mathrm{Sm}{}^{3+}$ ($Z=62$) and $\mathrm{Gd}{}^{3+}$ ($Z=64$)
3. C$\mathrm{Pr}{}^{3+}$ ($Z=59$) and $\mathrm{Tm}{}^{3+}$ ($Z=69$)
4. D$\mathrm{Nd}{}^{3+}$ ($Z=60$) and $\mathrm{Dy}{}^{3+}$ ($Z=66$)