I spent countless hours over the years studying the handful of pictures of Andy Chamber's Skaven army in White Dwarf 137 and experimenting with different ways to try to capture the feel of it. I never managed to replicate it, despite trying, and would love to hear in detail his technique and see it in the flesh or at least some better photos. I really envy the more loose expressionistic style of painting that some can pull off, but my personal style is too controlled and for some reason I can't shake it. So what you see is the result of this process of experimentation with many different techniques used, the army was painted over many years and is still being slowly added to.
The process I have currently settled on for the skaven is I prime the figures white, then give them a black wash to bring out the details.
Next I apply thin glazes of colour, starting with the metals, then the flesh, fur, clothes, straps and other details. The colours for the most part are quite random, whatever browns happen to be close at hand, apart from the cloth which I use a khaki drab colour that I mix differently for each batch to give a bit of variety. I think I use varying ratios of Vallejo Khaki, English Uniform, Russian Uniform, and Olive Drab
At this point the figure looks like a washed out pastel version of itself.
The next step is a bit hard to describe- I then deepen and darken certain areas by using layers of citadel washes Sepia, Devlan Mud, badab black, and ogryn flesh until it looks "right" ie there is adequate contrast and tone to the different areas of the mini. Before these washes came out I used oil washes, and in some ways I like the effect more but the citadel washes are way more convenient.
Generally the fur and metals receive more layers of the darker washes, and some areas receive no washes.
The metals I tend to leave as is at this stage. The numerous black and brown washes have given a used, rusty patina. I stipple on a watered down jade green wash to any bronze areas.
Then I drybrush the fur with a random lighter brown then the base, and then a very light selective drybrush with bleached bone.
Sometimes the other areas require to be highlighted by the base colour, then a final highlight layer of the base + bleached bone/white.
The last thing I do is paint designs on the shields, and some war paint on the fur by first painting the design in black, then going over it in white or bleached bone."