Relying on anti-freeze colour alone to determine it's properties is tricky. READ THE LABELS. OAT "long-life" anti-freeze (Dex-cool, for example) reacts with green anti-freeze, and depending upon which source you believe, will either cause a sludging effect or simply shorten the longevity (the manufacturers' opinion).
Here's a good chart with a breakdown of what is in what:
As BLjones stated - and he is dead-on - color doesn't tell us anything anymore. We need to know what the underlying chemistry is. A spec sheet will give clues. An MSDS will give clues. I make a part of my living developing engine coolant formulation - have for the past 15 years. Most are fine and seldom is the coolant the real cause. Really serious problems generally have their roots in 1 of 3 places:
* Electrolysis. A wire somewhere it shouldn't be, a loose ground. Standard boat stuff. Remember, coolant is an electrolyte, and when circulating it loves to set up a current, just like a moving wire.
* Exhaust leak. If exhaust is getting into the cooling passages, no coolant can handle the oxygen load. All will start rusting violently.
* Old-school high pH coolant where there is aluminum. Bad. Often old truck AFs fell in this area.
Sailingdog is correct about Dex-cool. Actually, it was a good formulation, as far as their testing went, but it is quite incompatible with even small amounts of dirt or other coolants... which makes it a poor formulation of course. They should have tested these scenarios, but they relied on pure lab data and "clean" test fleets. No real-word testing. I remember an experience with dirt early in the "long-life" development process in my lab; we had a new ingredient that was just great in every way and cheap too. On one set of trials I used some old dirty test coupons, just to see how it worked on existing surfaces rather than perfect lab samples. Total failure. The additive treated the dirt and the metal got holes. You have to test in the real world.
It is not just about Yanmar specs either; the whole design, including heat exchanger and pump, are critical. But often the boat installers are not savvy in this area. They just don't know.
The best bet these days, unless you are sure it has always had one type of factory fill, is one of the "hybrid" types that claim to mix with anything. I do not intend to name names, but the larger brands have tested many combination by now, and they are formulated to be flexible.
The real exception to this generality is a large heavy-duty diesel with wet-sleave liners (not Yanmar - truck engines and the like). These engines have special needs and are well served by heavy duty NOAT products by Penray, Fleet Guard and others.