If you are using an air brush just spray one of the colors higher or lower than you intend then when you spray the second taper off the spray as you approach the line you wish to hold. Spray very lightly one of the colors over the other. Use a very light spay. You can go over the line as much as you want to get the fade you want. In general it is easier to cover a lighter color with a darker. With something like camo I spray the entire model the lighter and then spray the darker over that free hand if you want the diffused line camo. For sharp line camo I make a template (stencil) which covers the area, then spray the openings. A tight-fitting stencil gives sharp lines while a looser fit will give a bit of fuzzyness to the line.
As always practice on some scrap or as I do get some free styrofoam resturant carry out boxes.