I just did a gelcoat job on ole Undecided (removed some old ugly rusted out footstrap bolts) and I used the stock West Marine gelcoat. It came out a bit more brown/yellow once it fully cured than the white of the rest of the boat.

So get the color matching kit for like $40 and do thorough testing before you actually lay down the final coats.

If all else fails, then contact Sher-fab and ask them if they have records of the white that performance cat/Nacra used.