Question

Why are imines colored?

Answer 1

Some are, some are not. Diazo compounds are typically strongly coloured, for reasons which I will try to illustrate.

Explanation:

A diazo compound is typically an aromatic compound of the form `R-N=N-R'`, where `R="aryl"`. Because of the conjugation (i.e. charge transfer can occur across the aryl rings at an energy low enough to coincide with the visible spectrum, rather than the UV), these materials are strongly coloured. The discovery of diazo compounds, cheap, colour-fast dyes, caused a revolution which propelled many industries. Brilliant colours were available cheaply, without the labour of collecting sea snails or whatever.