Basic construction is the first thing a judge will look at. There should be no seams, glue marks, or sanding scratches. Parts and joints should be filled where necessary. That is, if you are going to have some panel lines or joints showing, be sure and do it the same way over the entire model. Paint should be smooth and uniform, with no smudges, runs, or fingerprints.
Colors can be almost anything, especially for scratchbuilt fantasy spaceships.
Models from television and films should try to match the on-screen appearance, where ever possible. Exceptions always exist, so judges should not make a big issue over color. Weathering, if done at all, should be subtle and to scale. Rocket engine nozzles should generally have some sort of weathering, particularly on the inside. Photos of the Apollo vehicles in space show no weathering on the outside of the Service Module main engine, but the exterior of a Shuttle main engine shows multiple tones of black and metallic gray.