Two sides to every story -- Sainsbury's talks of her being "aggressive" over the phone, so perhaps while she was using the word "coloured" just because that's what was said in her youth, it's possible that in the context of the phone call it could have come across as racist. In my experience older people tend to be more comfortable with speaking bluntly and directly, so she might not even have been meaning to be aggressive, either.
Either way, a blanket ban with what appears to be no willingness to talk about it seems an over-reaction -- although on the other hand it's frustrating to hear her describe it as PC gone mad. "Coloured" is a word that you shouldn't use for the reasons the DM gives towards the end of the article. I think the better way for the operator from Sainsbury's to have handled the phone call would have been to point this out "I'm sorry but you can't say that" and hopefully leave it at that, although even then it might have got out of hand.
But we'll surely need to hear the full recording to know what was said and how it was likely to have been meant?