I hope I'm not too late to the party. If I may, I'd like to tell you how I do multi-color spinners.
First, like the previous posters said, paint a white base coat. On that, spray the lighter of the two colors. After that, I mark the demarcation lines with a fine line.
Then, I use regular ol' electrical tape...
Cut into very fine strips.
The reason I like to use electrical tape is because it stretches a bit more, and is a bit more forgiving, than other types of "paper" tape.
Once my strips are cut, I cut one end at a sharp angle before I lay it down. I find that the sharp angle helps elinimate a "step" when you lay the finishing end of the tape over the starting end.
If necessary, after your strips are laid down, you can "fill-in" the rest of the area that needs to be masked with any other masking medium you like.
Just to give you an idea of how well this method works for me, here are some of the spinners on a few of my Mustangs.
Hope this helps. If you have any questions, I'd be happy to answer 'em for ya. Good luck!