paint mode - color erase mode

[gimp-artist]

   
I'm testing "color erase" mode for paint tool. Background color is cyan. I painted blue color on it in the "color erase" mode. Then the green color came out. It is fine.

Now background color is yellow. I painted red color on it in the "color erase" mode. I'm expecting green color for the result, but nothing happens. What's wrong with this approach?