Yep, red top is a normal starting battery not for deep cycle use
Yellow top is a deep cycle battery not for starting use
Blue top is a deep cycle battery capable of starting duties.
Here's where my confusion starts. Probably just the difference between what the Marketing department says, and reality, but one of the advertised advantages of the Red Top Optimas (at least when they first came out) was that they could take a drain much better than a regular starting battery. Which made them (according to their ad-type) more like a Deep-Cycle battery.
They still call the Yellow Top a "deep cycling and cranking" battery. Right on their home page.
And I can't see any reason why a Blue Top deep-cycle version, or a Yellow Top can't be used as a starting battery too. Seems to me that, just because it's not taken down to the minimum every time you use it, it shouldn't have a shorter life expectancy than a regular starting battery. They've got plenty of CCA's to get the job done (less than a Red Top or a Blue starting model, but still plenty) and have more reserve capacity. And I know of plenty of people running them with no problems. So far at least.
There is a warranty difference though, so that's something to think about. The Reds are the only ones with a decent warranty. Whether it's honored or not, it's much longer than the pipsqueak 12 or 18 month warranty on the Yellows and Blues. A little longer on the Blue starting maybe, but shorter than the Red. Guess that's due to "expected use" of the others, but it's too bad.
The Blue Tops are always a "Marine" style battery (with the dual top posts), but they're available in either a Deep-Cycle (I should say, "dual-purpose") OR a Starting version. Not one battery that does both, but different batteries with different part numbers. And then to confuse things, they don't even call the version of the Blue Top that I have, a "deep-cycle" anymore. They call it a "Dual Purpose" battery on their website and right on the label. So the Blue is available in either "cranking" or "dual-purpose". Go figure.
The Blue Tops are what I have now, simply because I liked the small threaded top posts that I could use to easily attach accessory wires to and still have a deep-cycle model.
Turns out that getting a deep-cycle version might have been a good choice, since they've all spent most of their lives just sitting most of the time!
Paul