Tad Spurgeon is an American artist and paint craft savant whose extensive website summarizes years of dedicated research into the ‘secrets’ of the Old Masters. An ongoing study of the materials and techniques of oil painting. Definitely a must read for anyone interested in oil painting.
Paintmaking.com offers useful information about various pigments, sorted by color and comparisons between ancient and modern pigments.
Bruce MacEvoy‘s extensive website presents in-depth information about pigments, color theory, watercolor techniques and more.
MITRA – Materials Information & Technical Resources for Artists. Also check their Useful Links page.
Kama Pigments based in Montreal and founded in 1996, Kama offers some of the highest quality paints and pigments money can buy. President and founder Vincent Deshaies is dedicated to the highest standards of quality and it really shows in his products. All tube paints are ground in walnut oil none contain fillers, binders or any kind of additives.
Kremer Pigmente based in Germany with a branch in New York is a supplier of quality artist materials with a HUGE range of pigments, oils, varnishes, chemicals, you name it.
Natural Pigments offers basic art supplies as well as unique and hard-to-find items. Based in the USA.