The problem, Teffy, is that we're not talking about what you see in-game. We're talking about the decks that you see posted here in the forum. In that case, you really do see every card in the deck, and you really can just look at them and say "OK, that's what this does."
That's why casting cost really is the only reasonable way to judge the mono-ness of a deck. Casting cost is the absolute determinant of what quanta the deck NEEDS to produce to function. And the rule really is simple:
1 elemental casting cost = monodeck.
2 elemental casting costs = duodeck.
3 elemental casting costs = triodeck.
4+ elemental casting costs = rainbow.
No complexity at all. No counting cards to figure out if something is primary or tertiary. All you gotta do is look at the corner of the card, and if you don't see a little elemental symbol there, it doesn't count toward the number of elements in the deck. Activation costs are ignored because by and large the deck can still do something without activating a specific off-element skill, but it can do nothing if it can' t pay it's casting costs.
It's DEAD SIMPLE. And it works perfectly.