Here's a decent primer on glass repair, albeit, working from the inside
Working from the outside, all the same principles apply, except that you're doing gelcoat (or paint) last, instead of first, and unless you're repairing a hole, you don't need the backing plate. To do a blind patch of a hole from the outside, you would insert some sort of backing plate from the outside (cardboard is actually OK) and either bond it to the interior of the hull, mechanically fasten it, or hold it from the middle with something you'll remove after the repair sets up, then just fill & fair the holes.
In either case, the wide grind area is important (feathering), as is building up layers of glass. It's the glass that provides the structure, the resin is just there for the bond, you actually want as little resin as possible.
All this is assuming you have a rather large repair, where structural integrity may be compromised by the damage. If you've ground out more than say, 1 1/2 to 2", I'd be putting some glass in there. For smaller cracks, just fill with gel paste or West and an appropriate filler. Microlight would be my recommendation in this case. Spend the $3-$4 to buy West's guide to understanding the characteristics of their different materials. A Dremel is good for grinding out small cracks which will likely just need filler. If you've broken out the angle grinder, you're probably talking about adding glass...
All my boats (power & sail) are so old that I'm not terribly concerned with cosmetics, so I'm not your best advisor on color matching! I'd be willing to bet that you could mix the 2 part paints to get the right color match though. A quick call or email to International will give you a definitive answer. As long as it's the same type of paint, it's seems perfectly logical to me, to do so to get the right match.
Color matching is more of an art than a science. I have an old school chum who is a 3rd generation body shop painter. He's amazing, can nail a color match like nothing, while it may take me 4-5 tests to get it right. Keep in mind that most paints and gelcoats will "dry down" a couple of shades darker. It is best to test small amounts on some cardboard or something before you commit large expensive batches of material to the actual repair.
And personally, I wouldn't bother with a mil guage, but I'm more of a seat of the pants kind of guy. If you're an analytical engineer type, you may want one. Three to 5 light coats of gelcoat is about right IME. With paint, just follow the mfg's recommendation.
In either case, if you're going to sand to blend your repair with the eixisting surface, make put on an extra coat or two. If your final cosmetic is to paint or gelcoat the entire hull, just make it as fair as possible for a nice finish, and again, follow the mfg's directions.
Looking forward to seeing pics!