sounds like you are not letting the colors dry before adding new colors. Sometimes that is what you want because you can then blend the colors, or do a drag thru multiple colors to make different effect like the flame cookies.
But if you want your colors separate; take a look at this blog - it will explain it better than I can. http://www.kingarthurflour.com/blog/2011/11/29/let-it-snow-let-it-snow-let-it-snow-globe-join-the-snow-glove-fun-and-make-your-own-cutter/
Yes, you must let each color dry before adding any other color right next to it. You do not have to outline in black as the link page said. YOu outline w/the same color you are going to 'flood' with. Keep your outline thin - don't use a tip any bigger than a 2. I almost never used a tip - just use a parchment bag w/the tiniest hole in it. It should take very little time for the outline to dry. So if you are going to use say, green, yellow and red, you do all the sections of one color, then move on to the next color that is as far away from those sections just flooded as the design allows. Then do the last color - working any areas that are not next to any you have already done. The drying time depends largely on 2 things: how large an area you just completed and the temp in the area you are working in. Of course, smaller areas will set-up/dry quicker than a whole cookie top :) Hope that helps.
I agree with letting the colors dry before moving on to the next, but it doesn't always work. I just finished a set of cookies for my SIL. Her favorite colors, and my least favorite to work with...red, black. Even with letting the cookie dry before moving on, the black still bled into the white. So I decided to run a test. I put one cookie in front of a fan, and kept another on the counter. The cookie that was in front of the fan had minimal bleeding, and the one that was on the counter....well, I just ate it lol
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