Hi Leester,
If you want a fairly decent "semi" line, there is a technique I will try and describe that works great.
I use tin foil to mask off with, it's .25 a roll and you will not get any leakage through it.
First I tape the tin foil on "backwards" like I was going to paint the other color, then pull it back sorta loosely over itself. This gives a distinct edge that is not quite where the tin foil is. A tiny bit of overspray will get under the tin foil (I only leave less than 1/8th inch and it is not really "up" off the plane) and you will not get a "sharp" edge to the division of colors.
Wish I could take a good picture of it.
Bill <><