How to change just one color in an image in Keynote

Last year, I had a project, where I needed to change just one color in an image. Keynote had a fantastic tool to do so, and it made it very easy for me, once i figured out the steps. It's been a year, and I forgot how.

It seems like the edit mask or remove background features have changed, and I'm completely befuddled at how to change just the one color. If anyway has helpful tips it would be appreciated. The only article of note I have found is the following but it is more about removing colors, not changing them.