Does your TV have more than one SCART input? If yes, try the other one.
What you describe is what happens when you plug the cable in a socket that does not support RGB. Most TV's only support RGB input on the first SCART socket.
Edwin
Same thing could happen if there isn't the proper voltage on pin 16 of the SCART connector. It should be between 1 and 3 volts to tell the TV that the signal is RGB.
