The appearance of a painted colour depends partly on the pigment, but also on the ingredients
used to bind the paint together.  
Pigments
mixed with oil produce a glossy finish.
Pigments mixed with egg yolk, to make egg tempera paint,
will have a flatter, matt appearance.
Pigments made from mineral clays, known as
green earths, bound in egg tempera, were used
by Italian artists from the 13-15th centuries
as a base colour for flesh.
These greens were used to suggest shadows,
and model the shape of the face.
Pinks, reds and white highlights were added
on top. 
Tempera dries so quickly, that different colours
can't be easily blended on the painting surface.
So artists used delicate hatching when applying
the different colours.
The very green skin tone of many Renaissance
faces may be due to the layers of pink paint
having faded, revealing more of the base layer
than was intended.
With oil paint, the pigment is bound in certain
kinds of oil, often linseed or walnut, that
dry to form a tough film.
Jan van Eyck was particularly famous for his
skill at working in oil.
Oil paint dries slowly and so can be manipulated
on the picture surface.
Some pigments are opaque, others can be
used to make a translucent 'glaze'.
Glazes can be built up in layers to produce
a deep, glossy colour that can imitate gleaming
satins, costly wool, velvet and rich silk damask.
