Dyes are basically ionising and aromatic compounds, they have Chromophores present in them. Their structures have Aryl rings that has delocalised electron systems. These structures are said to be responsible for the absorption of electromagnetic radiation that has varying wavelengths, based upon the energy of the electron clouds.
Chomophores make the dyes proficient in their ability to absorb radiation. Chromophores act by making energy changes in the delocalised electron cloud of the dye. This alteration invariably results in the compound absorbing radiation within the visible range of colors and not outside it. Human eyes detects this absorption, and responds to the colors.
Electrons may result in loss of color, their removal may cause the rest of the electrons to revert to the local orbits. A very good example is the Schiff's reagent. As Sulphurous acid reacts with Pararosanilin, what happens is that a Sulphonic group attaches itself to the compound's central carbon atom. This hampers the conjugated double bond system of the Quinoid ring, and causes the electrons to become localised. As a consequence the ring ceases to be a Chromophore. As a result, the dye becomes colorless.
To conclude chromophores are the atomic configurations which has delocalised electrons. Generally they are represented as carbon, nitrogen, oxygen and sulphur. They can have alternate single and double bonds.