As an artist, a good understating of the properties of pigments is crucial! In recent years, most manufacturers have adopted the good practice of detailing the pigment composition of their paints. Here you have some good resources to know more about pigments and colours:
Pigments in paintings is an online exhibition detailing the history of pigment use, with some curiosities, properties of the different pigments and examples of their use in paintings and other works of art.
The excellent guide to watercolour pigments at Handprint, with a review of all the pigments used by watercolour manufacturers.
Another good guide to pigments and their properties, richly illustrated with photographies.
In a previous post we shared a few interesting addresses related to natural pigments (earth, ground minerals, oxides...) Of course, there are several other posts here related to pigments, their properties and use.
In this french page you'll find a collection of slides with nice photographies and descriptions of a lot of pigments.