In the past I've always used either a black or white undercoat depending on the technique I was trying - black primer if I was primarily dry-brushing colours on to give shading and depth, white if I was going to do a black outline and fill the gaps, or more recently if I wanted a flat basecoat job to be shaded by Quickshade or similar.
These figures have been primed with Army Painter Pure Red Colour Primer, part of the Army Painter speedpainting system. This has a nice flat matte finish that the other colour paints adhere to very well. But by priming in the models' most significant colour (in this case the red jacket) you can save a deal of time by just painting the other colours and leaving the primer to show through.
Since this was the first time I'd painted these figures, I picked one to paint to conclusion, to get a feel for the sculpting style and pick up any potential pitfalls. That's the sergeant on the right, who's more or less finished and waiting for the Quickshade to do the dipping magic. If you look closely you can see I have indeed painted the checkered band on the glengarry. It wasn't too hard, although it's highly impressionistic and doesn't stand up to close scrutiny, it looks just fine at 2-3 feet away.
The general method I evolved when I was painting up for the Big Birthday Bash was as follows - split each unit of 10 figs into groups of 5. Officer and NCO figures I tend to do all in one go, much like the sergeant here. For rank and file I pick one part of the figure, e.g. the pants, and paint that on each figure in the group, then pick another e.g. the hands and faces and do the same. The order I do things varies by figure... sometimes it will be easiest to paint the brown and the black of the rifle before painting the flesh of the hands, othertimes a different sculpting style might make it easier to paint the hands and then paint the wood and metal around them.
Once I've got all the base colours painted onto the whole group, I put them down and do the next group. With both the groups of the unit completed, I then regroup them and go through the unit one figure at a time, fixing any faults I can see. This involves a lot of swapping paint colours and often I find myself having to tidy up a new problem caused at this stage e.g. If I've splodged some white from the webbing onto the jacket, I'll tidy that up with a stroke of red, but then I might find the correction makes the belt look too thin, so out comes the white so that can be repainted. And so on and so forth, making this a fairly laborious stage of the process.
Then all the figures have been "fixed", the unit's put aside ready to be Quickshaded, then the base painted and textured.
I do sometimes diverge from this routine, usually when I know I don't have the light or the right frame of mind to do a potentially tricky bit of painting. Like yesterday, with the light fading and feeling a bit bleurgh, I didn't feel up to picking out webbing and collars on the current batch of figures, so instead I just went through the whole batch of 20 scotties and painted their trousers. That way at least there''s some progress made on the batch as a whole. Today I might go back to the group of 5 I started working on, or I might just paint the boots on all 20 figs. As long as there's more paint on the figure at the end of the day than at the start, it counts as progress and eventually the batch will be completed.