"Paper chromatography" is a technique for separating and identifying mixtures that are (or can be) coloured. It has been largely replaced by "thin layer chromatography", but is still a powerful teaching tool. "Double-way paper chromatography", also called 'two-dimensional chromatography', uses two solvents and rotates the paper 90° in between. It is useful for separating complex mixtures of compounds having similar polarity, for example, amino acids. The compounds used initially ensures you to know whether the colour is pure, (one substance only) or mixtures (multiple substances.) 