The most important color–by far–is white. White oil paint comes in three flavors:
Zinc white (zinc oxide, PW4, usually called Chinese White when used in watercolors). Although known from ancient times, its common usage is relatively modern, dating from the 18th century when it was developed as a replacement for lead white, which was long known to be toxic. Unfortunately, it’s now well established that zinc white quickly becomes unacceptably brittle and can even peel and fall off paintings. For this reason and others I won’t use it in oil paintings.
Titanium white (titanium dioxide, PW6) became widely available for artistic use in the early 20th century. Titanium white is the most opaque and covering white. Because of that and because of its cool (blue-gray) tone, it appears garish unless used sparingly. Some artists combat the cold tone by mixing a small amount of a warm color, such as raw umber or raw sienna, into it before painting.
By itself, titanium dioxide forms a spongy skin that is unsuitable for artistic use. Manufacturers, therefore, mix zinc oxide or some inert material into their paint. Quality varies widely, so do your homework. Finally, while some claim heath risks with its use, at this time none of the claims have been reliably confirmed.
Flake or Cremnitz white (carbonate of lead). Flake white has been used by artists from the dawn of time and it is still the best white today. It’s less opaque than titanium white (a good thing!) and has none of zinc white’s problems. Additionally, flake white is a natural drier. This in itself makes flake white the King of Whites. Not only is the rapid drying of oil paint important in itself, drying agents affect oil paints in a fundamental way, imparting precision and ease of use at the same time. This last point is important enough to me that I don’t use anything but flake/cremnitz white.
So what’s the problem? The problem is that flake white is poisonous. Care must be taken when using it (I don’t grind my own oil colors because I don’t want to worry about powdered lead white). Painters have been using it from the dawn of time, like I said, and it can be used reliably with a bit of care and common sense–don’t smoke or drink when painting and wash your hands after use.
Another problem, related to the one just described, is that because of health risks many manufacturers have stopped making it. Blockx, a high-end manufacturer, no longer makes it. Luckily, many smaller, American manufacturers, such as Willamsburg, Blue Ridge, Doak, and Rublev, have taken advantage of this opportunity by filling the void with their own hand-crafted paints . I enthusiastically support all of them.
Finally, be aware that many paint makers offer paints called flake or cremnitz white that actually don’t contain any lead.