Shading can be done in two ways. The most common is to put a lighter version of the color to be shaded on the appropriate portions before laying down the main color. The second way is to put a clear type paint over the main color. As for highlights, you can spray or dab on a very pale color over the target area. Remember, masking is your friend. Also, in highlighting, it's not just mixing in white with the colors. It's about making steps in the color progression. In other words, to highlight dark blue, don't just use white, use a lighter blue
Okay, it's not really wrong, but if you think an airbrush is the only thing you need to get a professional looking model, then think again. A brush is just as important, because masking can't solve everything. An airbrush might give you a smooth looking texture, but it can't give you the curves and twists and freedom of a well guided brush stroke. In order to eliminate brush stroke leaks or ugly streaks, you might think of adding a little retarder to the paint (but not more than 10%.) This is imperative if you are using lacquer paints, but for Tamiya acrylics, quite unnecessary
Because the rest of the model is airbrushed, be very careful in painting the rest of the colors. You want to try to get that same smooth effect, and eliminate brush strokes, so take it easy and paint in delicate layers. Don't rush. It will ruin your effort up to now. Because of that extra layer of white, the color should go right on without resistance however
