Transparent preview?

Is there any way to change the colour/opacity of the preview in the glyph editing window from solid black? I find shapes easier to judge if they are not so high contrast. I realise that the background can be changed, but grey/pale green on a pale background works best for me. I’m sure it is a very personal thing, so an editable script or an addition to the User Settings pane seems worthwhile.

In Master settings you can add a ‘Master Color’ custom parameter.

Brilliant Georg, thank you. In a while such things will become obvious I imagine, but I hope that this thread might remain useful for other users tramping up the foothills of Glyphs.