Pigment is the part of the paint that is the colour. It is a finely ground powder that, when mixed with egg, water or other liquid, becomes paint.
The first pigments used were ground from earth and vegetable matter or minerals. Nowadays pigments are made chemically and are more permanent.
The study of pigments can be useful in determining the date of, and who painted, a particular painting.
Famous French artist Vincent Van Gogh used intense colours in his paintings to convey his mood and feelings. He was aware that the organic pigments he used would inevitably fade so he built up layers and layers of the most extreme colours in an effort to combat this fading. This can be seen in his painting The Church at Auvers (below) where he has used bold reds and and strong purple/blue in the sky.