There are more than just two colors of antifreeze/coolant out there. I know of yellow, orange, green, blue & red. I don't know if there may also be more that that.it matters little what material the containment system is made out of to either type. and it is the truth that the two type so antifreeze are NOT compatible with one another, although they WONT gel up if mixed,
A friend of mine had a very expensive repair bill from a Mercedes dealer, after loaning his car to a well-meaning relative who added the wrong coolant, so at least according to his mechanic, there is some combination of coolants that does plug up the cooling system. This was not a boat example, but some people do buy their boat antifreeze from the auto parts store, so I think that the example can be considered relevant.
The source indicated below, seems to state that certain coolants are incompatible with certain cooling system materials -
http://www.popularmechanics.com/cars/how-to/a91/1272436/
Quote from above link -
"The new organic acid antifreezes may be used only if the cooling system has an aluminum radiator (rather than copper-and-brass)."
My original information came from a conversation that I had, many years ago, with an engineer at a UAW plant in the Detroit area, where I was doing some technical consulting work.