The trick with any small scale paint job is to paint in brighter tones than you would use on a larger scale (eg 28mm). You need something to catch the eye and you don't have much surface area to work with. Thats not to say that you should end up with dayglo paint schemes but using bright colours in small areas of the casting should work well.
I paint a lot of 6mm and 2mm stuff and I tend to think that the undercoating is the most important part of the job. There are three options for undercoat, white, black and something inbetween. Let me explain a bit more of the pros and cons.
White allows colour to show well as light bounces back off the white undercoat. But if you miss part of the casting it stands out like a sore thumb.
Black tends to give a forgiving undercoat as any areas you miss look like shadow or at least that the theory. the downside is that it dulls down any top coat colours. Many painters seem to use drybrush techniques over black which speeds the painting up.
"Something in between", this is what I am playing with at the moment for 6mm figures. Choose the main colour of the figure or building and under coat in a light-medium shade of that tone. Dry brush over the top with a darker tone, ink wash with a darker tone. Pick out detail in bright colour and if you want it to really stand out (say shields) reundercoat in white before applying the colour.
I think those techniques should work OK on small scale buildings. However, the best advice is always to experiment until you arrive at a style that suits you, both in terms of the level of detail, the aesthetic of the figure/vehicle/building and the time you want to spend on painting. If your not enjoying the process of painting as well as the result your not doing it the right way for you.
