This is strange behavior, as when I use the button to change the outline color for all parts, they overlap the individual outlines. It's difficult to test because I don't have a Mac, and it works fine on Windows. If anyone else is reading this and has the same issue on Windows, please let me know in the comments.
The color picker does indeed zoom in when you're using the tool in full screen mode. I used Godot's native color picker and couldn't find an option to disable this feature. However, if you use the tool in default window mode, at least when using the eyedropper button, this issue doesn't occur (if you manually resizing it also works). I'll try to fix this in future updates.
Thanks for the feedback!