[Coco] Reverse video mod

[Joe Grubbs]

Steve posted pictures of his old CoCo2s, one of which as a Lowerkit and "Reverse video mod", both selectable via chassis mounted toggle switches. The reverse video mod gives green text on a black background. 

Does anyone know what is being done in hardware to make this happen? Is it separate from the Lowerkit? Steve mentioned there might be a blurb about it in an old issue of Rainbow, but I've not yet found it.

Note that this is something I'd like to do on a CoCo2 with the 6847 VDG (NOT the T1 version, which is apparently more configurable with some clever POKES).