Paper chromatography is a method for separating dissolved substances from one another. It is often used when the dissolved substances are coloured, such as inks, food colourings and plant dyes. It works because some of the coloured substances dissolve in the solvent used better than others, so they travel further up the paper.
A pure substance will only produce one spot on the chromatogram during paper chromatography. Two substances will be the same if they produce the same colour of spot, and their spots travel the same distance up the paper. In the example below, red, blue and yellow are three pure substances. The sample on the left is a mixture of all three.