I own three different whites: Zinc White (unexceptional but non-toxic), Titanium White (of very limited use but cheap) and Lead White (great stuff, but expensive and really poisonous). They differ in their composition, in the effect they have on the other colours they are mixed with, their drying times, handling and legality.
This is the granddaddy of whites, used from when oil paints were invented. It is made from a Lead Carbonate (2PbCO3·Pb(OH)2) and various oils (usually linseed oil although occasionally walnut oil). It has a thicker, stickier feel than the other two whites but flows comfortably with a little extra oil. When mixed with other colours it does not kill their chroma quite as much Zinc or Titanium White. Lead White is often sold as “Cremnitz White” and “Flake White”. When made with linseed oil, Lead White has a tendency to yellow if placed away from light. It is expensive. In the UK, it is supposed to be sold (with a health warning and a lot of small print) for restoration purposes only and, because it is toxic, in one or two countries it is illegal.
For painting portraits this is the only white that really works – it brings greater transparency to other colours when mixed with them compared to Zinc and Titanium White and enables artists to keep a vitality in lighter skin tones.
This is made from Zinc Oxide (ZnO). It flows quite well on the brush but has a tendency to cool colours it is mixed with. It is a very slow drier which some artists appreciate as it means they can correct their work in a more leisurely way.
This is made from Titanium dioxide (TiO2). It has a particular brilliancy in its pure form and is usually the cheapest of the three whites. Like Zinc White but more so, it tends to cool and adulterate other colours, giving a pastel feel to them, a bit like adding milk to a cup of tea. It is useful for painting skies as its deadening effect on blues and greys is less obvious. Also, as skies usually cover a large part of a canvas and Titanium White flows very easily, it is quicker and more convenient to use a Titanium White mix than with a Lead or Zinc White mix.
Above is a photo of the three whites mixed with a vermilion. You can get an idea of the general and relative effect each of the whites has on the colour. From left to right Lead, Titanium, Zinc.