I agree with the subject of the bug report; what I didn't agree with was the specific suggestion of warning about --color, but it's true that currently there's no warning in the man page. Something like (adding a paragraph to the GREP_OPTIONS section):
You should consider the effect of GREP_OPTIONS on other programs that use grep: avoid options like `-P' which will change their behavior. (Shell aliases are a better way to include such options in a default set, rather like rgrep.) Programs calling grep should unset GREP_OPTIONS to avoid unintended behavior (and security holes). If we can get an agreed wording, I'll put together a patch for the man page and texinfo. -- http://rrt.sc3d.org
