Yeah, Alans right. You need to workout if it's the TR board, or the Video distribution card that's at fault.
Also, if you have a small monitor, with an Svideo input (yellow) you can use this input to view the green output in Black and White.
But as Alan has suggested, play a setup, then swap the small RGB output connectors directly on the TR stack.

So does the RGB monitor "Green screen" In non-TR mode, it redirects the black and white primary output (same as on the back) to the Green Video.
Atttached, is the SCART diagram (thanks to the person that originally uploaded this, it was very useful info :-)