Bruce.... Every time you mix paint you take a chance of a color difference because of the mixture differences. That not only means when the color is mixed, but also when it is mixed to spray. If what was left in the can when you stopped was not properly mixed/stirred up. Also consider the color of the primer that was under the paint on the quarter panel. If the primer was light in color, and the color of the paint that you are painting over on the extensions is dark, the finished color will be darker. Sounds like you may need to strip the extensions, and start over.
I hope this helps.