I've got tens of thousands of miniatures and only perhaps a couple thousand of them or so are painted at any given time. I used to have more painted minis but I sold off several of them in the late 90s. Since then I've been better at buying miniatures. I'm a pretty fast painter and I'm usually looking for nothing more than a decent table top standard on my finished minis since I am painting them to game with.
Recently I've been using a glazing technique over a black and white under painting to achieve suitable results. The dragon above is painting using this technique. Although it is not the best result I have achieved using this technique it's the miniature I used for this tutorial so I'm showing it here. I think it turned out fine for the purposes of this demonstration. I've always felt Green Dragons should have a plant like feel. The dragon's spikes were supposed to look thorn like so I think I managed that with these results.
You're going to want to use a drybrushing technique with the medium grey for the first layer. If you want the miniature to have a lighter over all result you will want to cover more of the black and only leave a little showing through. If you want a darker over all result then leave more of the black showing.
I usually keep my brush damper during the first stage of the under painting because you want to cover more of the black base coat but you don't want the brush too wet. If the paint is getting down into the deep crevasses you need to clean the brush and start over with a dryer brush. I'm using Delta Ceramcote Hippo Grey here.
One thing I can't stress enough when doing any drybrushing technique is that you need to clean your brush out frequently. In terms of drybrushing you don't want your brush bristles to get dry and too gummed up with paint. This is usually the downfall of the drybrushing technique. When the brush gets gummed up with paint it is hard to control were the paint goes and it will also damage your brushes.
After you put the medium grey down move on to your light grey. This will form the base for your highlights so you want this grey to be fairly light. I'm using Delta Cermcoat Drizzle Grey as my highlight. Here I have painted several minis at one time. Since this is a speed painting technique it works well for rank and file even though these are all different minis.
Make sure you don't go too heavy handed with this layer. You want to pick up the highlights but you don't want to completely cover the medium grey. You will want even less paint on the brush for the highlight and you want to avoid letting the brush gum up so wash your brush frequently. Just make sure you get the bristles dry after washing. You might want to employ a hair dryer on the lowest setting to help dry your brushes after you wash them.
I like glazing with a soft round but not a super soft one. Your looking for something with flexibility but not so much you loose control of the paint. I like soft synthetic bristles. Rather than stiffer hog hair or softer sables.
You can employ glazing with other painting techniques. As you can see above I have painted the miniature's teeth in before moving on. I use either dry brushing or layering on metallic surfaces such as armor on humanoid miniatures and sometimes when painting flesh tones.
After my glazes have dried I use Games Workshop Citadel "Agrax Earthshade" wash over the entire miniature to help tie it together and provide some additional shading. I got a little heavy handed with this model. You can thin the wash down if you don't want it as dark. The wash will help fill in the areas where your different color glazes touch and help blend them together.
There are some ares like eyes and bases that I don't paint until the end. You may also want to add some final highlights with dry brushing or layers as needed after the wash dries.
That's basically all there is to it. It took me about 2 hours to paint this dragon which isn't too bad. I've painting 28mm character models in 45 minutes or less using this technique so it can be really good when you need to get something on the table fast.