I've worked it out. If anyone's interested I did change mathed/math_support.C such that
diff mathed/math_support.C mathed/math_support.C.orig
555c555
< {"textrm", LyXFont::ROMAN_FAMILY, inh_series,LyXFont::UP_SHAPE,LColor::math},
---
> {"textrm", LyXFont::ROMAN_FAMILY, inh_series,LyXFont::UP_SHAPE,LColor::black},
Regards,
Paul Stansell
