FlyingBlindUK,
Have a read of this link to a
submission to the Australian Human Rights & Equal Opportunities Commission in 2002.
It is a submission from an experienced air traffic controller in Australia who suffered discrimination at the hands of the authorities as a result of his CVD.
While I'm not aware of what the outcome for him personally was, his experience again highlights that colour defectives are just as capable at performing tasks as our colour normal colleagues.
Just like the CVD standards for pilots, I would suspect that the standards for ATC's are founded on baseless assumptions of risk rather than actual evidence and experience.
As others have mentioned, make sure you go and get a formal test done from an optometrist before you get too worried. Don't just trust the ones done online!