A set of 3 cards plus a rules card given out to backers of The Dice Tower 2018 Kickstarter campaign.
"Take 1 cube of the depicted color from the bowls and upgrade 1 cube (this upgrade may be applied on the cube you just received."
"Trade 3 yellow cubes for 1 brown cube or for 2 red cubes. If you have 6 or 9 yellow cubes for trade, you can get a mix of brown and red cubes (each trade of 3 yellow cubes is independent)."
For more information, visit the BGG listing.