I am not well versed in Belgian beer history, and what you said about the different styles may be true from a historical sense. However every Tripel I've had to date (whether from a Trappist, or non Trappist Belgian source, or brewed elsewhere in the Belgian Style), has always been a light colored but strong brew. So unless someone was drinking beer out of old historic bottles from some cellar, or had found some obscure currently produced beer labeled as a Tripel that is dark, he couldn't really make a statement like "The darker tripels, like this "Ommegang" from NY I drank last night are a little too much for me right now.".
To strengthen my argument, take the fact that Ommegang is not even listed as a Triple on either ratebeer or BeerAdvocate. The correct style category for it is Belgian Strong (Dark) Ale. In a perfect world unclear designations like Single, Dubbel, Tripel, Quadrupel will vanish and clear qualitative style titles like Belgian Pale Ale, Belgian Dark Ale, Belgian Strong Pale Ale, and Belgian Strong Dark Ale will replace them, But based on current practices, these terms are somewhat interchangeable, and the shorthand titles are sometimes more common than the proper descriptive style titles. So when I order a Tripel, get me a light strong beer, otherwise it's going back to the bar. If I wanted a dark beer, I'd have ordered a Belgian Strong Dark.
Cheers.