What do the different bands of colour represent on a chromatograph?
In short, because different colours represent different chemical compounds, which differentially adsorb onto the chromatograph.
The stronger the adsorption of a particular pigment to the chromatograph, the longer the retention time of that pigment, In ideal circumstances, a chromatograph accomplishes a separation of the components into individual species. Of course, the experimenter has to choose what solvent and what paper to use to effect separation. If the individual species were really valuable (say one component was a cure for cancer), you could cut up the chromatograph into strips (with a pair of scissors!) to get the desired species as a single component.
A video Explains in details the chromatography paper.