Question

Why the halogen color deepens from Cl to I?

Answer 1

The colour of the halogen is due to absorption of visible light by their molecules resulting in the excitation of outer electron to higher energy levels. Fluorine being smaller in size absorbs violet light (of high energy) for excitation and appears pale yellow while iodine absorbs yellow light (of less energy) for excitation and therefore appears violet black In between, the colour of chlorine is yellowish-green and of bromine is reddish-brown or orange.
Thus, the colour deepens down the group.

Answer 2

the color is because of absorption of visible light by their molecules in their excited state