I am trying to find a color with "Find Image on Screen". The colors are desktop backgrounds (red, blue, green) that are visible when Mission Control is enabled.
Use a display-text-in-window action or a notification to make sure your macro is actually working.
Chances are you've missed a step somewhere.
When taking your screenshot with ⌘⇧4 you need to hold down both the Control and Option keys.
This copies the screenshot to the clipboard for pasting – and prevents the system from adding shadow effects to the screenshot (which can foul up find by image).
(At least that's how things work on macOS 10.14.6 Mojave.)