Halogens are coloured because of there low ionization energy, thereby the electron gets excited by absorbing visible region of light. Reflected light thus impart the complimentary colour to the molecule.On moving from fluorine to iodine, the atomic radii increases, so the energy levels becomes closer and the low energy light (higher wavelength i.e. towards red) is absorbed and colour deepens (complimentary is violet) on moving down the group.