Undo a mistake

How do you undo a mistake? I can't seem to make the pickaxe work. My first puzzle seems to reach a point with the last several choices that you either have to guess or you have to do the color change for a hint. I thought these puzzles were supposed to be solvable with no guesses. The color change button is confusing to me, not quite sure about its use. I'm probably just being dimwitted about all these points. Hope someone can enlighten me.

Figured out the color thing. I had never done a multi color one before.

All Nonogram puzzles can be solved without any guesswork. If you make a mistake in dragging a line, you can shift the mouse to the next row/column to cancel it without penalty.