color of unicode-math in xetex

Question

My question is based on the example which may be found here in teh second answer. Actually, I discovered this feature of unicode-math some time ago, but suspected that I am doing something wrong.

Setting the color of the math font using \setmathfont changes the color of all letters and symbols but not of the horizontal lines in fracions, square roots, etc. It is rather unexpected behavior. The question is, whether this is a bug or feature. And, if it is a feature, how to change the color of these elements?

Minimal strangely working example:

\documentclass{article}
\usepackage{fontspec}
\setmainfont[Color=000000]{XITS}
\usepackage{unicode-math}
\setmathfont[Color=FF0000]{XITS Math}

\begin{document}
A bunch of text, then an equation.
\[
 N^2 = -\frac{g}{\rho_0} \frac{\partial \rho}{\partial z} ,\;\;\;\;
 N = \sqrt{-\frac{g}{\rho_0} \frac{\partial \rho}{\partial z}}
\]
\end{document}

Produces the following result for me:

Answer 1

XeTeX font color feature only colors glyphs of the specified font, intervening non-glyph material is not affected, and the fraction and radical rules here is an example of such material (they are horizontal rules drawn by TeX and not font glyphs).

To color the whole equation you have to use general text coloring packages.

Answer 2

I use this to create white equations with transparent background:

\everymath{\color{white}}
\everydisplay{\color{white}}

Edit: the empheq package is not necessary, my bad.