[XeTeX] Change fonts for different environment/commands

[Arthur Reutenauer]

> In a fact I'm looking for such solution that is compatible with my
> existing projects - I don't want to create new commands and
> environments. Is it possible?

  Yes.  The general solution for this kind of problem is to save the
original meaning of the macro using \let, and to reuse it later, such
as in:

	\let\originalfootnote\footnote
	\renewcommand\footnote[1]{\originalfootnote{%
	  \addfontfeature{Color=666666}%
	  #1}}

  I didn't test this definition, but it gives you the gist: \let
declares that the macro \originalfootnote has the meaning of \footnote
at that precise moment (and won't change if \footnote changes), then
\renewcommand redefines \footnote using its original meaning.

	Arthur