The lanthanide ion that would show colour is:

The elements or ions having unpaired electrons show colour. The valence shell electronic configuration of ${La}^{3 +}$ and ${Lu}^{3 +}$ are $4{f}^{0}\text{and}4{f}^{14}$ respectively. Hence, they are colourless. ${Gd}^{3 +}$ is also colourless, this is because, here $4f$ orbitals are half filled,hence stable.As such $f - f$ transitions are not possible because they will destroy the symmetry of $4f$ and make them unstable. ${Sm}^{3 +}$ has the valence shell electronic configuration $4{f}^{5}$, hence, it is coloured due to the presence of unpaired electrons.

$L{a}^{+ 3} =$ Colourless
$G{d}^{+ 3} =$ Colourless
$S{m}^{+ 3} =$ Pale yellow colour
$L{u}^{+ 3} =$ Colourless