Smudgy image on monitor

About three years ago I gave an old computer to a woman who had been kind enough to let me store some of my “suff” in her shed.

She spent hours playing Windows solitaire and then moved on to Tetris. After a period of many months enjoying Tetris, she telephoned me to say there were smudgy images all over the screen.

Before I got around to calling on her, someone else had fixed the problem. I ascertained that this person had eliminated the problem by altering the display from 256 colours to 16 colours.

This is something that would never have occurred to me. I would have just done a full reinstal. Can someone please explain why changing the display eliminated the symptom?