I use Scotch Magic Tape for masking.
It is best to prime your models before painting them, and absolutely essential if you're painting yellow or white. I use a light gray paint for priming and use only enough paint to barely cover the model and make it all the same color. You don't need complete coverage for primer. For yellow, I'll follow the primer coat with a very thin coat of white. Again, not complete coverage, just enough to ensure a uniform color before applying the desired finish color.
As far as the sequence of colors is concerned, just think about which color would cover the other one easier. In general, the darker colors cover the lighter colors using less paint. The idea when painting is to use the least amount of paint possible to gain complete coverage. The best way to achieve this is by making multiple passes with thin coats. As each thin coat dries, the following pass adheres and covers the model better with less paint. So the accumulative effect of four or five thin coats is thinner and more complete coverage than one long heavy pass. The concept is the same whether using rattle cans or an airbrush.
If you have a complex masking job to do, you might want to sketch your design on paper first. Place the sketch under a piece of clean glass and lay out the tape on the glass. Cut the design using a sharp razor blade. Remove from the glass carefully and apply to the model. Use a toothpick to push the tape into all the nooks and crannies.
Hopefully that will give you some ideas to progress. I don't claim my methods to be the best, but they work for me.
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