When using changeobject it HAS to be the active window – there is currently no way to modify object properties on a non-active window.
I think @JamesCook is actually talking about changing the color of an Image Display object in particular, using the formula in that object as described on this help page.
I recently noticed a problem similar to what @JamesCook is describing, I thought it was something that had gotten broken in the Apple Silicon transition. Perhaps it goes back farther than that. I will investigate.
Update: Looks like this is a problem with recent versions of macOS. It fails on Big Sur, works on Sierra. I think this may be the same issue that caused problems in the data sheet when moving left/right in a row.