Sell your work to make cash.
Auction off your pottery for "coins" (in quotations because they're not ACTUAL coins — wouldn't that be nice, though?). Save your virtual cash, and you'll eventually make enough to buy more colors and patterns for your future masterpieces. The best part? None of this requires the use of your credit card, and believe it or not, that isn't the case for many other gaming apps (something I'll never get on board with). Tip: take a screenshot of your sculptures before selling if you want to look back on what you created, because there's no way to revisit your sold artwork.