Thinking about it for an hour produced a decision (partly based on previous impressions, and partly solidifying and crystallizing those impressions) of green/blue, and I was off
Its initial cost is three (three green) with an additional X where X is how much mana the controller pays
That's no different for the Umbreon VMAX Alt Art from Evolving Skies, which has climbed to a staggeringly high new average selling price
However, as per WotC's comments in the Banned and Restricted post, the need to ban Up the Beanstalk also comes from current and potential future design issues with other cards
Sometimes, a subject's popularity is more important for a card's value than anything else
Here, Ill only describe what in my opinion works best, leaving out the multitude of less successful trials