[Coco] Strange video issue

[Barry Nelson]

I built this circuit and installed it in my CoCo 1. 

http://www.cocopedia.com/wiki/index.php/File:Trs80coco2compmod.gif

It mostly works, except for pmode 4… On a pmode 4 screen, what should be blue shows up as greenish turquoise, and what should be reddish orange is instead reddish purple. Any idea why or how to fix it? Otherwise the video is ok on most other screens.