I wish they had a better description of that. I'm synesthetic, with normal color vision, but sometimes get sensations of colors that seem impossible to experience. 'A kind of greenish-purple', for example - no, I don't mean blue, and it's not a purple pattern with green bits, it's green and purple at the same time.

I also get 'colors' that make even less sense. For example, I'll occasionally get a color that 'looks' grey but doesn't 'feel' grey; it feels like it should have a separate label, and my mind refuses to categorize the stimulus with things that evoke 'truly grey' reactions. That makes me wonder if I'm experiencing the same effect as the one mentioned in the article.