If it's no too late, you may want to free-hand the red line (it can be thicker and/or sloppy, doesn't matter right now) and let it dry. Go over it with some Future and let it dry, again. Then, use a tape mask to cover over it. Seal the edges of the tape with a bit of future and let it dry again. Then spray with your yellow (may need a few light coats to get decent coverage). When the yellow's dry, remove the mask and you'll have a nice, even, red stripe.
Now, as for why this method will work better - yellow has poor coverage and red is notoriously hard to cover. Your best option is to lay down as little red as possible and then mask it off. The mask is what will give you the sharp edges. The future is to keep it from pulling up and the future on the edge of the tape is to keep the yellow from bleeding under.
-Fred