Yeah def can still change it! Whenever you place a tile or melt a facet, you get one "bomb credit". It's the bar in the bottom right. Whenever it fills up one of the facets chosen randomly on you unplaced tiles becomes a bomb. So it depends on what tiles you have left unplaced. I think a strategy would be to clear out big chunks of a certain color, and leave behind a different color, if possible, to try to influence what color becomes a bomb next.