Why are halogens coloured?

Why interhalogens are more reactive than halogens?

Transition metel compounds are generally coloured. Why?

Define inter halogen compounds?

Why is copper sulphate pentahydrate coloured?

A blackish brown coloured solid 'A' when fused with alkali metal hydroxides in presence of air, produces a dark green coloured compound 'B' which on electrolytic oxidation in alkaline medium gives a dark purple coloured compound 'C' Identify A, B and C and write the reactions involved.

What makes alkali metal halides sometimes coloured, which are otherwise Colourless?

Account for the following: Halogen are coloured.

Why La^(3+) ions are coloured?