While I do see your point (and the actual rarity levels/colors are never explained), this was the last planned update.Thank Faerin.
I would like to suggest a feature.
It would be great if the cards actually had the rarity word written on them
Or make the sphere icons much larger and include a letter in each one for the rarity. Such as a C inside the grey to indicate common
And I don't think it's important enough to warrant another update (in fact, you're the first to bring this up).
Personally, I would argue that a player either chooses what they think is the best card (in which case rarity is not relevant) or wants to obtain all cards for the bonus event (in which case rarity doesn't matter that much either, as you will always be offered a card you don't have yet, regardless of rarity).
So all in all, I guess the rarity could just be dropped altogether