Can I paint my car, a panel at a time, over a period of a few weeks? I've been told that I will experience color matching problems, but I don't understand this if I'm using the same reducing ratios and everything is protected from the sun.

You can, but there are a couple of drawbacks to painting in this manner. This is an operation where the sum of the parts are greater than the whole. It takes more time to paint individual parts, than to paint the complete vehicle. I would suggest assembling the vehicle first, ( sometimes assembling later damages the paint) and doing some meticulous masking. Make sure the car is prepped to your satisfaction. Get everything ready, and leave the painting for one Sunday. The actual spraying of the car will take you about two or three hours. Then another hour for cleanup, etc. The rest of the day you can spend admiring your work, or wondering how you are going to fix any imperfections that may have occurred.