To be honest, I've been painting for almost 35 years...and still found this slightly challenging to do correctly (gas tank)...or at least to my standards.
When using decals from factory, or reproduction....the color separation can actually go anywhere under the decal. In my case, the decal is a blue stripe, and has a pearl white stripe next to it. So the small pearl white stripe goes against the red, and the blue goes to the white painted area. I tried to make the red separation as close to where the decal goes as possible, just to keep the raised paint edge from showing through the decal. When I did the tank, I took a different route, and left it all white. Then put the decal on, then painted the red right to the edge of the white stripe on the decal.
So basically, the tape/stripe kit keeps the lines nice and sharp.
If your doing something custom, I can show you that as well....but you need quality masking tape either way.
Below are pics when I was painting my Aprilia RS250 about 6 years ago.
during painting stripes
http://www.apriliaforum.com/forums/atta ... 1326425398" onclick="window.open(this.href);return false;
after it was all done
http://www.apriliaforum.com/forums/atta ... 1326425332" onclick="window.open(this.href);return false;