You could use the outline you already have (which presumably is correct for your bus), but in a different package - Paintbrush (or Paint) is horribly basic. If you use something like Photoshop that allows transparent layers you will be able to avoid erasing the underlying outline picture, and you could also have different paint and graphic designs on the go at the same time, and compare them easily and quickly by just switching layers on and off.
Sign makers and people who do car graphics with vinyl cutters use vehicle outlines for all the car models you could ever think of that come on CD-Roms. The CDs can be bought from Ebay etc, but probably wouldn't include bus outlines, and they are designed to be used with vector-based sotware (Corel Draw etc, or proprietory sign making packages) which is probably harder to learn than more familiar bit-map packages such as Photoshop. The specialist paint shops and graphic people who do a lot of bus and RV work will undoubtably use this approach, probably with outlines they have produced themselves, but I suspect you will be better off using the more straightforward Photoshop method with your existing outlines
Hope that helps
Jeremy