Kellen, Daring Traveller is played in decks that do not play Green

[bakert]

But it adds a green stripe to the mana bar.

We could:

(1) Exempt adventures like we do Trap cards and others.
(2) Try and come up with something more general for "this card has two or more costs" and only include colors in all the costs.
(3) Exempt Kellen specifically.
(4) Inspect the "produces" of the other cards in the deck to see if any of them make the colors in the cards and create a super general solution that also excludes things like reanimation targets that can't be hardcast.

I like (4) now that we have produces but it might be overkill as the (2)-style solution probably does 90% of what we care about.