Quote:if its possible to paint over an already painted model
Yes, in a way. It depends on what you want to do. If you want to just switch around colors, then absolutely. Just change the RGB values in whatever paint program you are using. If you want to completely repaint the aircraft in a different style and different colors, that's a bit harder.
Quote:is it possibly to overlay a pic of a real plane and kinda of trace it on?
I suppose you are referring to just taking the image, pasting it over the existing one, painting the textures and then erasing the image you pasted in. Is that correct? I suppose that would work, although it might just be easier to paint it normally, as the textures might not be in the shape you are thinking of.
For more information on texturing, click on the repainting aircraft link in my signature.