There are times when painting a two tone car I just don't get the paint line I like where the two colors meet. One of two things happen... either there is slight bleeding under the masking tape or the line is not as clean and smooth as I would like even after sanding and cleaning up the paint line. That said, this is where decals can come to the rescue and provide the clean line breaks you're looking for and add a little "pop" to the car.
In the pictures below, the first picture is of the car with just the paint lines and how they meet. their not bad but just not the results I'm looking for. On both cars I used a decal stripe and covered the paint line with the decal. The second picture of each car you will see the strip applied and the results it provides. First it covered the line and provided a clean paint color transition. Second if you choose the right color, it will help tie the colors together. You will also notice two different cars but I'm using the same decals on each.
Anyway, hope this will help give you another option when painting a two color car. Think Decals to help with the color transition!!