Hi Blair,
Wonderwoman is correct, contouring is a very skilled art which takes a lot of practice to achieve using a range of light and dark solutions. I guess it's possible you could learn the basic theory about contouring online but the rest would be trial and error. With regards to using an HVLP gun, this would only be possible for certain contouring techniques. To achieve the 300 style abs like Wonderwoman mentioned, you would definitely need a precision airbrush, absolutely no chance of achieving this with an HVLP gun. The solution comes out the nozzle at too fast a rate and the spray pattern is too wide, not precise and would drip all over the place.
In short, there is no best solution for contouring, all solutions can be used for this service. Choose your favourite brand, the larger the range of percentages available the better and take it from there. You will definitely need an airbrush though.
Good luck!
St.Moriz Salon