Dan addressed the bonding issue, so I'll take a crack at the color issue. You're correct that there could be a difference over time. If you're going to be applying multiple fill coats after bonding the deck and hull, you may be able to get away with mixing epoxies IF you sand them carefully and evenly, but that's likely to be difficult. I would hesitate to do it, as the boat could end up looking streaky or splotchy over time.