Chromatography is the process of separation of solutes that dissolve in the same solvent.

Ink is a mixture of 2 or more colours (dye) dissolved in water. Different colours present in ink have different solubility in water. The component which is more soluble rises faster in the strip of filter paper. The component which is less soluble travels less distance in the filter paper. This is why different colours present in the ink rise to different levels during paper chromatography.