As all we know colours and paints are two different things. And even white colours can be different because of pigments. Let’s take a look at main white paints.
Up to 19th century the majority of painters used lead white (P.W. 1). The main advantages of this kind of white are high opacity and shorter drying time throughout entire layer of paint (for oil paints). However, white lead is extremely toxic and “thanks” to this feature even nowadays a lot of artists mistakenly believe that oil painting is toxic.
Another disadvantage of white lead is darkening if contacted with air, especially in watercolours and gouache, where pigments are least protected by binders. Also white lead doesn’t work for fresco painting due to its reactions with alkaline medium.
Zinc white (P.W. 4), another kind of white, was discovered in 18th century and gradually replaced white lead. This pigment has high light-fastness and works very well with most binders. From safety point of view Zinc white is not toxic. However, Zinc white is semi-transparent and dries slower than white lead. Also it absorbs more oil (in oil painting) which may cause some cracking after drying passing the time.
Titanium white (P.W. 6) was introduced to painters in the USA and Norway 100 years ago. Titanium white is non-toxic and can be used even in food packaging and as a food colour (E 171). Also it’s opaque and has high light fastness. However, its drying time is much longer than of zinc white. Due to its high opacity, Titanium white may turn colours dull and “foggy” when mixed with other colours, especially with cobalt. Titanium white doesn’t work for fresco painting because it’s too opaque and covers the base instead of mixing with it.
Generally speaking, for mixing colours Zinc white works better. For opaque layers, high-lights and for painting in light shades we’d recommend using titanium white. Some manufactures prepare mixture of zinc and titanium white to combine benefits of both in one single tube.