Hi Simon, I use acrylics mostly and I paint most of the time from the midtone colour upwards and downwards - usually doing the shadows first, then the highlights.
To give a simplified example we'll say we're painting a brown that can be highlighted by adding white and mix shadows using black, I would mix at least two highlight and shadow colours (usually more):
medium highlight - brown + white, 1:1
light highlight - brown + white, 1:2
medium shadow - brown + black, 1:1
dark shadow - brown + black, 1:2
I paint the item the basic brown over the primer in a couple of thin coats, as many as necessary until the colour is uniform, then paint in the medium shadow followed by the dark shadow. After that I'd do the highlights in the same way - medium highlights first, light highlights last. That's the most basic way of painting something in acrylics, in oils you can also work this way although if you are blending on the figure (painting 'wet on wet') the method can be very different.
In acrylics you avoid hard edges between each colour by not painting with the paint at full strength, you thin the paint down a lot and build up coverage in many applications. Although the paint is very watery you avoid it going on like a wash (where it would settle in recesses and not stay where you put it) by having very little of it on the brush - the hairs are damp, not wet.
Blends are also made easier if you have more than two highlight and shadow mixtures, three or more means each colour is less different from the previous step so it's easier to make a smooth transition from one colour to the next.
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By the way guys, it's not a good idea to do this:
http://bragonart.free.fr/images/bragonart/...ge/tirer_04.jpg
Einion