The term halogens refer to the elements present in group 17 of the periodic table.
Halogens are coloured because they absorb different quanta of radiations that lie in the visible region.
On absorbing these radiations, the electrons get excited from the highest occupied molecuar orbital to the lowest unoccupied molecular orbital.
The energy between these HOMO and LUMO decreases from fluorine to iodine.
It is because of the fact that the amount of energy that is required for excitation between these levels depends entirely on the size of the atom.
Smaller element requires a large excitation energy because of the greater force of attraction between the nucleus and the outer electrons.
This large excitation energy is sufficed by absorbing a shorter wavelength (violet light) and hence, on relaxation it gives a pale colour.
Whereas, larger atoms need lower excitation energy and hence absorb lights of larger wavelength. These, thus, appear deeper in colour.
Final Answer:
Halogens are coloured because they absorb and emit in the visible region. The colour deepens down the group because of increase in size and hence, the decrease in energy between HOMO and LUMO.