From @bjorn :
The easiest for now would be to keep the rendering there, but to pass in the fillColor from MapObjectItem::paint

Sorry but I couldn't quite understand how MapObjectItem::paint connects to the drawMapObject methods. I see the line where you set the opacity to 50. And I see your example of reading the type values I believe here:

    // See if this object's class has a color associated with it
    if (auto type = Object::propertyTypes().findClassFor(effectiveClass, *this))
        return type->color;

But I don't quite get how to set the fill color in MapObjectItem::paint so that it would be used in drawMapObject

JSON export worked and import seemed to as well, but when I reopen Tiled it doesn't seem to be automatically getting this value correctly so I probably missed somewhere where it gets initialized.

Updated editor with toggle - might be awkward to have this on the "Use as" line so let me know if you want me to move it somewhere else

I'm thinking maybe to the right of the color at the top right would be a better place for the checkbox

This is partially working now, let me know if it's not what you had in mind.
One thing I need to find where to implement is saving and restoring the drawFill property from the project file. When I reopen Tiled after setting drawFill to false it default back to true with my current version

Test map/project
drawFill.zip

eishiya raised the question of if this might be better served as an opacity slider rather than a boolean checkbox, for some more flexibility. What do you think?

I was actually mostly there on restoring the property from the project file. I just was missing an assignment.

Example screenshot of the feature on an orthogonal map

Seems there's a bug though -- I might have done something wrong to cause this - when I first open the test isometric map I made, there are objects drawn in the wrong place. Mousing a tool over them clears them.

Also eishiya and I noticed that this isn't working for "point" type objects. I guess those go through a different code path. Not a huge deal for me but I can change it just not sure what I missed.

An example project
drawFill_multiple_maps.zip

• Respect this setting for Point objects as well?

While I think point objects do need some customization, I don't think they need to respect this fill setting.