In case anyone is reading this years later like I was years. For me it ended up being the power switch. I just switched it on and off several times and eventually it went back to colour.
NOTE: Just in case these were important - It went Black & White after I'd had the machine on for several hours (playing Zombie Near). It came right while trying to get an Atari game running in colour as suggested above (Secret Quest). I turned it on and off a few times and one of the the times it came back in colour. So, after unplugging the Atari module, I cycled the on/off switch several times until colour returned. I'm not sure if any of the other steps had any influence on that. But my suspicion was it was just a matter of turning the machine on and off a few times.