Well, the mist is watered down food coloring. It is going to run or "bleed", no matter what you do.
Air brush machines, depending on what kind you get, are better at "painting" on cakes than just plain bottled mist/spray colors. Everything is adjustable (or should be) on the air brush, so you can control what happens better than just spraying from a bottle or can.
Air brush machines can be expensive, and you can get some cheap ones off ebay, but the really good ones usually start around $200.00.
Of course then again, unless you are using this machine several times a week, it will be a waste of money.
If you use stencils, you might look at using food color paste and artists brushes. You have more control and the color is in paste form, so you can control how much you use and how it is applied to the pastry.
Either way, as with all things, practice makes perfect.