[NTG-context] mathcolor attribute in MathML (mkiv)

Hello list. I'm trying to use colour in some MathML formulae using the mathcolor attribute. It seems to have no effect on the output. I can see \applymmlmathcolor in the x-mathml.mkiv source but I can't see how to make it work. Does anyone have any pointers? (Does lmtx support this?) Thanks. Duncan