https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/222575/dg_067666.pdf
That's the legal guidelines, so follow that and you'll be fine.
I would say that no, they're not road legal because the document states that the characters must be black (bottom of page 8). By definition, the carbon effect characters are black AND grey due to the 'weave', so they would not be road legal.
Would you ever get picked up on it? Highly unlikely. Could they? Yes, they could. Your call on the risks.