I have a mountain and an undercity sewer. I float red black, as well as two colorless mana from mightstone and weakstone. I then activate Candelabra of Thawnos, targeting the two colored lands. When paying the cost for x=2, I get asked how to pay and can click the two colorless from mightstone to pay. The next turn I do the exact same play except instead of undercity sewer I use a swamp. This time, after clicking the two colorless mana to pay for candelabra, Arena used one colorless and one black instead, without the option to go back and rewind. This cost me a game since I needed 2 black (I even had dark petition in hand as the only card needing red or black) and the game decided to use one black instead of the colorless I had clicked on when prompted