Change image foreground colour in Pavlovia

I’m making an experiment in Psychopy 2021.2.3. Ultimately, I would like to change the colour of grayscale images dynamically so participants can adjust the colour. I’ve seen here it says you should be able to do this so long as the image is luminance only: ImageStim — PsychoPy v2022.1.3

However, when I try to change the ‘foreground’ part nothing happens. How do I check if the image is sufficiently luminance only for this purpose, and why might it not be working?

Thank you!