Some problems are matching the color, and if you are on tour drying time, and how to apply it.
If the color is right, nail polish is better than car paint in some respects. It's very hard and dries very fast, and comes with a nice touch up brush, it is the easiest to apply. I have had good luck color matching nail polish on most bikes, not all. The stores I went too allowed me to take the bottles outside to hold against the bike for a color match before purchasing them.
The 12 year old nail polish touch ups look the same. Car paint needs to dry for at least a day, longer if it is humid or cold. Some of the Testors paints will dry very well overnight.