Take the needlebar cover off and take it to your favorite paint counter and have them match it. Likely the smallest amount you'll be able to buy is a quart, but it's relatively inexpensive, and you'll have plenty for touchups!
Miriam, the problem with the 500a is that it's aluminum, which means you have to use a special primer on the bare metal after you get the bubbled stuff off. It's a project, that's for sure!