Customize math font (serif, color, etc.) in an environment

Question

I'd like to define a math environment that behaves exactly like flalign*, except (1) the font is all red, and (2) the math is all rendered as if it were wrapped in \mathsf

I've tried:

\documentclass{article}
\usepackage{xcolor}
\usepackage{amsmath}
\begin{document}
\newenvironment{myenv}
{\color{red}\begin{flalign}
}
{\end{flalign}}
\begin{myenv}
Foo
\end{myenv}
Bar
Baz
\end{document}

But this turns everything red after the environment, not just within it. And I don't even know how to set the font for the whole environment.

Is this possible?

Answer 1

You can't define environments based on amsmath alignments that way.

See also https://tex.stackexchange.com/a/670933/4427 for sans serif math.

\documentclass{article}
\usepackage{amsmath}
\usepackage{sansmath}
\usepackage{xcolor}
\SetSymbolFont{largesymbols}{sans}{OMX}{iwona}{m}{n}
\newenvironment{coloredflalign}{%
  \color{red!90!blue}\sansmath\csname flalign*\endcsname
}{\endalign}
\begin{document}
\begin{coloredflalign}
A&=B & C&=D & u&=v \
x&=\sum_{i=1}^n a_i & y&=M\otimes N & z&=U^v & x\ne z
\end{coloredflalign}
\begin{flalign}
A&=B & C&=D & u&=v \
x&=\sum_{i=1}^n a_i & y&=M\otimes N & z&=U^v & x\ne z
\end{flalign}
\end{document}