why is prontosil colored?

Answer 1

It is the azo bridge between the two phenol groups that is responsible for the color. Many such Azo compounds exist, and a good number of them serve as dyes.

The color is caused by the absorption of light in the visible spectrum by the π-system.

Answer 2

Because it has the grouping C6H5-N=N-C6H5 in it. This is called a chromophore (color bearer). The conjugation of alternating single and double bonds around one benzene ring through the -N=N- azo group and into the other benzene ring interacts with light (absorbs on the blue side) to produce a red color.