Hi, @ShellyG, I think you're directing this question to me. (Do tag the person you're addressing, as otherwise that person can miss the question. Thousands of people are on this site and read this area, and I don't see every post.)
But to your question! Yes, I do a lot of multi-coloring stenciling. I do this primarily by airbrushing very close to the stencil (1 to 2 inches away) with limited pullback on the trigger (aka limited coloring flow), so that the coloring can be directed into very small areas with minimal overspray into areas you don't want it. I only mask off adjacent areas if the areas are very, very close, or if blending of the two adjacent colors (which invariably happens) will lead to an ugly color (sometimes the blending looks cool and very natural). For masking, I most often lay down my Quadrant Masking Tool on top and weigh it down with small magnets, but, again, I only take this added step if necessary.
I have several videos showing me stenciling with many colors on my YouTube channel, where I review these techniques in detail. You may want to check out my stenciling playlist there.
Also to note: That girl with umbrella stencil is not my design, and it is not my stenciling in the image that accompanies it here (https://www.confectioncoutures...ray-cookie-stencil-1). It is a design by my stencil partner Confection Couture Stencils, and the image they show is a graphical representation of what coloring might look like in that stencil, which is why it looks so perfect and clean. (If it were a real airbrushed cookie, there would be some blending between colors unless they masked off every area between colors.) Conversely, all of my stencils - which are the Prettier Plaques and Dynamic Duos lines only - are accompanied by real cookie photos where I personally did the stenciling using the techniques outlined above. My cookies still look pretty good (if I do say so myself ), but there is always a little color blending (oftentimes intentional) at transition points.
I hope this helps. I won't be checking email or posts here until after December 27, so I can reply more fully then if needed. Happy holidays!