[NTG-context] Colour (color) in mathml

Hello. I'm trying to implement part of a change-tracking system using colour to highlight changed areas in a document. I use MathML for maths, and I need to be able to apply colour to individual elements in an equation. I thought I could do just simply <mi color="green">x</mi> but evidently not. It does look as though colour is dealt with in x-newmml.tex, but not this way. Is there a way of colouring specific elements of a MathML equation? Thanks very much. Duncan