Neo,
I dont want to appear to be argumentative but a dark background can work. Look at the first place winner of the sc-fi category. I shot that on a black background and it looks fine. Many magazine editors require white backgrounds mainly because it is the easiest to edit into a block of text as well as being easy on the eyes and not harsh contrasts on the pages but white can be difficult to work with and takes some getting used to when photographing.Photo editing software of some sort is a good idea and will help with whatever color you choose. There are free ones on the web that work fairly well.
The all open category is tough, you could be competing against a build that took a year to build and won gold in the advanced category at a model show. Thats just the chance one takes in that category, last years winner took years to build and it was truly a work of art.
As to Norms point, rusty cars competing with shiney cars, its a no win situation for us. If we start drawing lines, where do we stop? We could easily spend weeks debating every possibility of finish/type/class/style/era/design/purpose etc. And end up with 200 categories and 185 entries amd then everyone will be a 1st place winner. Unless Scott gets his crystal ball back up and running, we dont know how many entries there will be in what categories until near the end so until then we will have to continue keeping it as simple as possible.
Your mustang got a lot of votes because it was a good model. I would suggest you stand proud of what you accomplished, it was a tough competition. Just stay the course and keep building, you can only get better.