Why are Lanthanides Coloured?

The unpaired F electrons in the lanthanides ions are the cause of the colour in this element.

Answer:

The colour of lanthanides ions is due to the presence of partly filled & orbitals. As a result, it is possible to absorb certain wavelengths from the visible region of the spectrum. This result in a transition from 4F orbital to another 4F orbital known as the F – F transition.
Such unpaired F electrons give colour to the elements.
Lanthanides ions are classified as “rare earth elements”.