One can read a lot of
HOWTO paint well
while browsing through the web.
There are pages, that advice you to prime in white and there are one which favour black.
There are pages that tell you how to shade, and how to hightlight.
There are pages on a lot of different subjects, but they only state personal opinions.
What I will try to do here, is make (kind of) scientific experiments with miniatures paint.
If you want to contribute, just send me a scan of your experiments.
I have used two kind of setup.
First there was a steel sheet, washed and primed with white spray paint.
On this sheet I tested various paints and methods of painting.
The second setup was a bunch of old goblin archers. They were cleaned up, washed and primed white with the same spray paint. This setup was used to investigate effects on a detailed surface.
Priming black or white?
I made a simple test with different paints.
As you can see I had one side painted in white (primed in white and then painted white), and one side in black (primed white and then painted black with GW chaos black).
Then I painted thick lines over both sides.
The colour was not dilluted and painted fairly thick.
The scan does not show the details very good, but the overall effect is clear. Black is not a good colour to paint on. On the other hand black is a very good colour to paint with. So instead of priming the miniature with black I would advice to prime with white and then get a big black brush and paint the parts, that you want to be dark.
The first is a "noname" blue colour. It is fairly bright when painted on white and quite dark when painted over black.
The next colour is a "noname" flesh colour. It is visible on white and black, but to get a good colour on black you will have to paint at least twice. As a synopsis to the above I would not prime in black, except when painting dark elves. Some friends of mine painted a lot of dark elves and priming them black works very good on them. The only other colours they used were silver, elven flesh and purple, which all work quite good on a black background.
Shading or why to be carefull
Washing is a easy and fast way to give a model depth and painting structured surfaces.
Upps, the last sentence was wrong in more than one way, as I will explain here. First washing is the method of painting with a stark dilluted paint ink with the intention that the colour will remain only in the recesses. There are a lot of tricks to improve the effect (using alcohol or blowing on the paint), but they don't work as far as I know. But before I will explain my thoughts I will show you some pictures. The first one is a model, primed and washed with GW blue ink. The second one is primed in addition with a white colour and then washed with a dilluted green colour. The third is primed in addition with white and then washed before the white colour had a chance to dry. As you can see the first model is completly blue. The ink had no intention to flow in the lower areas. The second model is a little bit better, but still a lot of the colour stayed on the higher parts and not much got into the lower. The third model is better insofar as the effect is very clear. Light green is on top, green in the lower parts.
In addition I made some experiments on a flat surface. The first shows a dot of blue ink on a primed surface. As you can see the colour is darker at the border, which is as you would expect it when you paint aquarells. The ink seems to dry in such a way that dryer parts get more ink. This is exactly what happens when we wash on a dry model. The lower parts get more colour in the start, but when drying the paint gets sucked back on the higher surfaces.
The next picture shows the same experiment but this time the surface was made wet before painting. As you could expect it now the paint dries even.
Synopsis: Washing is neighter fast not good. You should generally not just wash over a large area. The result cannot be controlled. When washing make the surface wet or better paint quick before the old paint dries.
The odd drying could be used for some effects like camouflage.
The silver effect
When I painted silver it got spotty and plain ugly, like brushed aluminium.
I couldn't explain this until I read somewhere that silver get's only shiny if you don't thinn it.
So I just tried it.
In the first picture you see silver painted straight out of the pot.
It looks quite shiny.
(Note: you have to stir metallic paints or the pigments will settle themself at the bottom.)
The next picture is my experiment to paint thinned down metallic. You see the effect described above: The paint will not be shiny. Instead it will look like brushed metall, quite a interesting effect on it's own, but not what you want for metal weapons and armour.
Synopsis: Don't shade with metallic colours. Drybrushing on the other hand seems to work quite right.