Pigments are coloured materials from rocks, plants, or chemicals, that dissolve in water. They are made in the form of cakes or tablets. When water is added, the powdered pigment becomes liquid again. The pigment is applied to white paper or white card by brush or cloth or cotton wool, and allowed to dry. Unlike painting in oil-based pigments (coloured materials dissolved in oil), a watercolour can not be painted over without spoiling the colour, whose purity depends on the white backing below. Therefore each area of a watercolour painting is painted only once, and looks fresh and original rather than worked over. Watercolour painting is often used in Chinese, Korean, and Japanese paintings. Fingerpainting with watercolours came from China.
Watercolour Facts for Kids. Kiddle Encyclopedia.