All the halogens are coloured and the colour changes with increasing size of the halogen molecule. It is due to

The colour is due to absorption of light in the visible region. The energy acquired in this manner excites the valence e^-'s to the highest occupied (HOMO) to the lowest unoccupied molecular orbital (LUMO); i.e., transition from ${\mathrm{\pi }}^{\ast }\text{ to }{\mathrm{\sigma }}^{\ast }$molecular orbital.