I'm trying to replace old ceiling light with a new one. New light has only blue and red, no green. There are 2 cables coming from the ceiling each with 3 wires. Green/blue/red and green/black/red. I've blocked green/green and tried blue/black to blue and red/red to red but light goes on but not off. Blue to blue and black/red/red doesn't work. Any suggestions? UK.
Green is indeed ground pretty much everywhere except old work in the Soviet Bloc.
You need to attach the new light the same way the old light was attached. It is likely of the 4 wires from the wall, two of them were attached to each other and not to the lamp. Reconnect those same ones the same way.
In wiring, colors don't mean a whole lot, becuase cables are made to standardized colors. (Old: black-red... new: brown-blue). So the information of how wires are connected is valuable. It is the only documentation for how the circuit is wired.
However, in that era in the UK, they were fond of using a special-color cable for "switch loops". In this case, the color coding sort of works out.
- Red is supply/always-hot, and comes from supply and goes to switch. It does not go to the lamp, the lamp has no use for always-hot.
- Black is old neutral, and should go lamp neutral (which is blue/EU or white/US.)
- what remains is switched-hot. It is Blue (old) from the ceiling. And from the lamp, brown/EU or black/US.