That is usually done after the photo is taken, by editing it in your computer.
But the Canon G9 (and some other powershots, not all) have selective colour mode.

I clearly selected a Grreen peg, and it left the yellow post-it note coloured.
So I would not rely on it, the best way to do it is still to shoot in full colour and then edit it in the computer.

But that selective colour mode on the scene (SCN) setting on the dial may be good enough for her.