Here is how I connect a composite TV or monitor to a DEC Rainbow.
I use the color monitor cable BCC17 and plug it into the mono port on the Rainbow. It leads to a breakout box. The keyboard plugs in there. I use the green/white BNC with a converter to RCA jack and plug into the composite of a TV or monitor. The display comes up but a little bit is cut off on the left. Go into setup, second screen and change screen width from 132 to 80. On some monitors a tiny bit may still cut off on the left but that can stillbe adjusted with the monitor's settings