Take the car to a body shop supply house, they can set a special camera on the gel coat and snap a pic, this calculates a formula for a near perfect match.
In the past I've mixed my own touch up paint when I didn't know what the paint code was. For example I had a off yellow kinda cream MGTD. I bought cans of Dupli-Color touch up, light yellow, off white, cream and a tan. Bought a Prevail self contained aerosol bottle and mixed in a bit of each color until I had a near match. ~Alan