I've found the gelcoat appears to change colour once the MEKP is added but once it hardens it should be pretty close to what you want. An old painters trick is to get a piece of clear glass and drop your mixture onto it (this is before the days of automatic matching) and look at ot from the other side and it would give you the finished product. It might work for gelcoat also although I usually just use a toothpick and spread it onto the surface I'm trying to match as I add tint, easily wiped away with some thinners/acetone.