I would suggest a few ways:
Almost Airbrush:
Do the next best thing to airbrush and use a darker base paint in a rattle-can.
Liquitex are the inventors of acrylic based paints and their spray line is impressive (~$15 per can, look for sale coupons like the Michael's 50% off):
http://www.liquitex.com/spraypaint/
http://www.liquitex.com/Resources/AcrylicLiterature.aspx?taxid=104
You can also get different nozzles for varying spray patterns wanted (~$8).
Fuzzy Brush method:
- Typically you should prime starting with Black, it tends to be a bit more forgiving unless you want really bright colours.
- A large flat head brush is pretty much a must.
- Water your paint down well so there are no abrupt colour transitions, needless to say, multiple coats are inevitable.
- Start your first coat for "block painting" slightly darker than the intended colour and make each pass slightly brighter avoiding recesses. If the base colour is to be black or a dark blue, start slightly brighter (a little toward grey for black) and then black or brown shade the recesses.
- Washes are helpful for also adding a gradient in the recesses.
- You may want to look at "artist inks" which is really a high-flow, high-pigment acrylic paint made to act like an ink. It could work very well as a wash for brush painting.
- Some have primed white and then "painted" with the inks which is similar to a watercolour method where if you are careful all the shading and highlighting is pretty much done in one or two passes.
- Dry-brushing does the same thing as washes but for raised surfaces.
Note: Get all your "block" painting and touch-up done prior to shading: any painting after the fact will look bad except for possibly edge highlighting.