First off, I'm very familiar with model contests. The main thing is to have fun building your model and learn techniques from more experienced modelers. Will be very helpful if the judges leave comments on your entry sheet if improvements are needed. What one judge sees as an issue another may not. I have had models not place at all in one show but then place 1st place in another show a short time later. One of these models was a German armored vehicle which I was certain to win at least a 2nd place but didn't get zip. About two months later I entered it in another show and won 1st place gold. I later found out that the reason my model didn't win the first time was because the individual judging that table didn't like weathered entries regardless how realistic the weathering was.

As for your color question, there is no magic formula for the "correct" shade. Manufacturers have slight differences with shades as it happens in real life with the real aircraft. Too many variables involving lighting, application method, coverage, weathering etc. will display different shades of the same paint. Best you can do is to approximate the color.

And yes panel lines are WAY overdone on models and most are overdone. Next time you are close to an aircraft check out the panel lines and you will see how thin and shallow most are. Dirt, oil and gunk gets trapped in these lines and sometimes makes them pop more than what they really are.