Once the paint is cured, you an apply any paint over any other paint. The trick is not to use excessive amounts. When I'm doing bead work, for example, I'll paint the white background color, then add the more colorful "beads" or patterns.
In genreal, paint lighter to darker, since you'll need less paint for coverage. However, there are times when you're doing small details wher that's not practical. Small details, such as dials, knobs, etc, can be easily brush painted.
Caveat don't try brush paintng with Tamiya unless you're an alchemist.