Why are la and Lu colourless?

Answer 1

Lanthanum (La) and lutetium (Lu) are colorless because they have unfilled f-orbitals, which do not allow for the absorption of visible light. Instead of promoting electrons from lower to higher energy levels, their electronic transitions occur in the ultraviolet region, beyond the visible spectrum. As a result, they do not impart color to their compounds or solutions. Additionally, the high symmetry and lack of significant crystal field splitting in their compounds contribute to their colorless appearance.