In the show you don’t know much about the origin of the General until the 7th season where you find out the Duke boys found it at a Police impound yard. They were looking for a car they could use for a race that started in two days. It had been wrecked and was a faded black color but they knew it was the one. After taking it to Cooter’s shop the proceed to work tirelessly on the car fixing the front end and installing an engine the boys had been working on. The iconic paint color, well that just happened to be all Cooter had left in his garage so that’s how it was chosen. This is also when the doors get welded closed because according to Luke Duke all race cars have welded doors, its a safety thing.