Sometimes the need arises to repaint an aircraft. Perhaps you're trying a new color scheme, or most often a part has been damaged and could use a new coat of paint to conceal a repair. A common practice is to take a part of the airplane into a Home Depot or other large hardware store, and ask their paint department to "color scan" the part. Their computer will color match the part with a special scanner, and output a matching paint mixture. A pint of water based acrylic paint costs $3 or so, and is enough to paint an entire aircraft with a coat of matching color paint. This is far more cost effective than buying a $3-$8 bottle of paint at a hobby shop, which may or may not be an exact match to your aircraft.
When applying the paint, ensure it is thin enough to run like thick milk. Too dense a paint will add unneccesary weight. Too thin a coat of paint will run and look splotchy. Apply with a clean brush in smooth strokes. Try to minimize the number of brush strokes. Allow the paint to settle and even out. This can reduce the appearance of streaks in your final paint job.
In the end you should have a touch up paint job that you can't even notice between the new touch up and the stock paint.