Here's what I do. I took some lines from a NWS11 base I liked. I changed their colors to fit my scheme then I merged them together. Use your rectangle tool and highlight them. Copy then paste to your new Gen6 car. All you have to do then is resize and move the lines where you want them.
Now if you're doing something that has lines that go from the front to the back of the car you will need carviewer to make sure you are getting the lines well lined up. Getting a paint scheme just right takes a lot of patience...I'm not very good at this either. I know there are many very talented painters here. For more information go to youtube and search "nr2003 gimp tutorial" That should give you some videos on how to paint in gimp for nr2003.