TeX math \color{} command in OMML formats

[d-morrison]

(from #10221): the \color{} TeX math command can't be converted; for example:

---
format: docx
---

$$\color{red}{x}$$

produces the warning:

[WARNING] Could not convert TeX math \color{red}{x}, rendering as TeX:
  \color{red}{x}
        ^
  unexpected control sequence \color
  expecting "%", "\\label", "\\tag", "\\nonumber", whitespace or "\\allowbreak"

[d-morrison]

just realized that I was supposed to post this in https://github.com/jgm/texmath; found an existing issue there (jgm/texmath#225) and added a comment with a link back here. I guess this Issue can be closed?