Thank you for the very helpful info on that alternative RCA Mod!
So, it would seem there was a much easier solution. I simply rotated the pot on the PCB a few degrees and the color magically apeared!
With some trial & error I settled at a spot where all my games look ok, with color on my TV. However the colors do look a bit different compared to the other Atari 2600 consoles I've modded.
By the way, does anybody have the slighest idea what that huge block of hard plastic encloses inside and why it's connected on to the PCB?!
I would like to X-Ray it!