There are some issues with the code. But before discussing that, I have my doubts about the idea itself. In my mind changing the grid color is not a frequent occurrence and therefore I would not expect to see this feature in the Draft_SelectPlane task panel. And why only the grid color and not the other visual aspects of the grid (transparency, colored axes, human figure...)?

This was mostly because when you change the UI theme, your grid color often needs to be changed too. And it takes a lot of tweaking to find the right color for a certain background color. When you need to go to the preferences, then restart FreeCAD to see the changes, this takes a lot of effort.

Now with this, you can much more easily tweak the grid to exactly the right aspect and intensity you want. The other settings are indeed not so important to set often, that's basically why I left them out.

Yes, the _param_observer_callback_grid function in params.py currently does not handle grid color changes. I missed that. IMO we should improve that function instead. And even if a button is added to the task panel, that function needs to trigger the change.

Okay for me

Thanks for the fix! This PR now depends on a fix in the params observer, right?

The param observer already triggered the correct functions. Not changes were needed there.