They leave the paint to dry, then re-wet it when they need to use it. Others purchase pan sets and when a pan is empty they squeeze tube paint into the pan and leave it to dry. Some brands, depending on their composition, respond more favourably than others to this treatment. Put simply, watercolour paints are coloured pigments suspended in a medium. In the manufacture of artists' paints.
Some colours are composed of single pigments, others contain several pigments.
The advantage of single pigment colours is that they don't separate in the tube, they don't fade at different rates, they offer superior lightfastness, and they can always be relied on to be exactly the same colour. Pigments are derived from various sources, natural and synthetic. Natural inorganic pigments compose the 'earth' colours such as Burnt Sienna, Raw Umber. They are derived from clays with high iron content, are very lightfast, fairly granular and will last indefinitely. Synthetic Inorganic pigments are mostly toxic metallic elements such as the cobalts, manganese colours, cadmiums, chromiums.