There are times when nail polish is about the only way to go for touching up very small areas. Some paints, such as white pearl , are virtually impossible for the home bike restorer to match. The solution is to get thee to a big chain drug store in a racially mixed part of town. There you will find at least a couple dozen nail polishes in white pearl. You can take a fork in as someone has suggested or a matching pump if you have one or do what I do: be very nice to the attending clerk and see if you can bring the whole bike in. I find a genuine friendly attitude will get you (and your bike ) into some surprising places.
I think for most of us, touch up is only right for very small areas. Really big areas require a Pro. One tip is to look at the color you want to match and see what is available in Rustoleum or DeRusto. It is available in a surprising array of colors, makes a wonderful primer or is easily tinted and it has the wonderful attribute of sticking to just about anything and of letting any more closely matched top coat stick to it. A great many touch up paints are finicky in this area. I'm not sure if this is drifting off topic or not, but I'd be glad to give a few more pointers off list to anyone wanting to try touching up bike paint. It took me many years and some good tips from Lou Deeter and others before I was happy with my own results, but the techniques are all very simple.
Tom Sanders
Lansing, MI USA