To keep acrylics from drying out too fast, add a small amount of retarder to some decanted paint—don't put the modified stuff back in the bottle, either! For tiny amounts and touch-up, dip your brush in a thimble of clean thinner (whatever is appropriate for the brand of paint you'r using) before dipping it in the decanted paint.
"Provided you have a keen eye for mixing colors, there is no real difference what so ever." —Mike S.
Mike, as a professional illustrator, this is a skill you had to acquire fairly early in your career, and it is not an easy skill for many, perhaps most, to acquire. I once color-matched paint for a living, so I have an idea of how difficult it can be. But I agree that for figures, it could be a good economical approach. I just wanted to add a caveat so that someone doesn't try it once and give it up as too hard. It takes time, practice, good color vision, and good light—basement modelers take note! But it's a good skill for a modeler to learn.