Hi Les
This is my method. It works really well.
Visit local paint / decorators trade supplier who has paint mixing facility. Choose colour cards which approximate to gelcoat colour. Take to boat and choose exact match. Return to paint supplier with colour code and small screw top container. Ask to have just the pigments added to small container( no paint base).
Choose white epoxy / gelcoat filler which also has white catalyst.e.g plastic padding.
With a little experiment a touch of pigment added to the filler will give a perfect match.
Alternatively if you have cause to cut a hole in the cockpit take a small sample of fibreglass to paint supplier who will place in a colour analyser to produce a unique pigment mix.
Good luck
Lee