The elements or ions having unpaired electrons show colour. The valence shell electronic configuration of La3+ and Lu3+ are 4f0and4f14 respectively. Hence, they are colourless. Gd3+ 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. Sm3+ has the valence shell electronic configuration 4f5, hence, it is coloured due to the presence of unpaired electrons.
La+3= Colourless
Gd+3= Colourless
Sm+3= Pale yellow colour
Lu+3= Colourless