@tool script says function does not exist when it does
In Godot GDScripts, if you try calling a function on a non-tool script from a tool script, you’ll get the error that looks like this:
res://test.gd:8 - Invalid call. Nonexistent function 'get_color' in base 'Resource (SwatchRef)'.
The error message here is misleading; the function could indeed be defined, but functions will not be loaded unless the script the function is in is also a tool script.
To fix this issue, use the @tool annotation on all scripts that contain functions you want to call.
I consider this issue to be a Godot crime.