No need for a "perfect" line. I use the paint brush method. Dip into fluid bed white, dip into the second color, hold above the area where you want it, tap,tap,tap. Distance from jig will vary with how big the area is you want to paint. For those lips I do in orange, it takes me 3-4 dips into the jar...just the tip of the brush, quick tap to get the excess off, hold id about 1/2" from the lips. To do the top part a second color, bigger brush, load it up, hold about 4" from minnow head. Takes several dips of the brush. The more detailed, the closer I am, and the less paint on the brush. Less is better.....you can always add more. I also keep going back and forth between the paint and the air gun when needed.
Are you using a fluid bed? A fluid bed with the second color in it will leave uneven lines from the ionizing of the powder, and the contours ot the minnow head. I hope I explained this well, I'm better in person lol.