I like the first part of your theory (that each Shard has its own color) a lot, but I think the Triumvirate inference is unsound for one simple reason: Parshendi and parshmen are known to have skin marbled in red, black and white. Therefore, it is immensely more probable that the "wall of red, black, and white" is referencing an assualting horde of parshmen/Parshendi.
I would venture to guess, by the way, that Devotion's color is either silver (because Elantrians turn silver) or gold (to fit the "Red and Gold" quote, since we know devotion and Dominion are complements).
Though this theory does seem to hit a snag with Endowment, who seems to be related to every color at once.
Also, I do not agree that "Shards each seem to have a number associated with then. Honor, for example, is 10, while preservation is 16." I think there is a different explanation for the numbers, given here: