I start with a grey primer (Tamiya usually), then apply a wash of slightly thinned self levelling black (GW badab black wash). Once dry, I go over again with black to make sure the edges between layers and in dark recesses are totally black. I then apply a drybrush of light grey, followed by white to pick out surface details further.
Tamiya Primer, Badab Black, Homemade Black Wash, Elric
The result is a preshaded grey figure, which shows off all the sculpting. I can keep on my shelf and admire the model until its time to paint. By priming, the model is protected from lead rot, so I am in no hurry to paint the figure- which takes off the pressure a bit.
Reaper Chronoscope Biker showing preshading undercoat
Now if your a black undercoater, you will find this method allows you the same benefits- namely you can leave a little paint exposed at the edges between surfaces for a nice lining and skip bits in deep recesses. However you will find this method gives you a few advantages over black undercoating. Firstly, you can see what your doing, secondly you get bolder colors. A finally you get to be able to use tints to quickly paint your model.
For tints and base color glazes I use matt medium (Vallejo model color is great) and a little water to thin the pigments out- making the paint more translucent without making it runny. The shading will shine through, quickly giving you a game ready finish. You can then go over this later, adding shading and washes to raise the quality of the finish.
Preshading and tinting works extremely well on fabrics, leather, pouches and bones, especially if you use a flesh or bone instead of a grey drybrush. This shines through a brown tint giving a great worn edge finish.