you can just fudge it. That’s what most implementations do, fade in the image, which often uses multiple colors and hardly resembles the nonogram at all. It’s a bit hard to interpret what your objection is, honestly.
Valid point, but my knee-jerk reaction to that statement is that I don’t want to fudge it and don’t want to swap out what they’ve solved for the larger collage I have them building.
A puzzle having more than one solution can end up just being a constraint that I apply to myself while making puzzles: every puzzle I make must have one solution. Then solving by solution or solving by not breaking contract with hints doesn’t matter.