You've got a good gelcoat supplier nearby - SherFab Supply (http://www.sherfab.com/About.asp). They supply the gelcoat for Performance Catamarans and can mix to color codes. You might want to call them about getting a white base with 3% gray...but it's going to be hit or miss as no two "whites" are the same. I would just go with a standard white for repair and feather it in - it would hardly be noticeable.