The two images attached show a grey background with a cyan circle and a black circle. There is a green rectangle behind, so green means transparent.
I want to make the black part transparent with the following filter:
object.fill.effect = "filter.chromaKey"
I test with two grey backgrounds: one has rgb(100,100,100) and the other had rgb(100,101,100). The rgb(100,100,100) grey turns green -- it gets chromakeyed along with black, which is unexpected. When I tweak the color a tiny bit (rgb(100,101,100) then it works!