A request for allowing users to customise font colours has been requested before and declined. IMHO, rightly so. Considering how some users abuse bold and code markup to highlight portions of text which they think is important, the thought of adding font colours to their bag of tricks makes me cringe.
As for customising the colours used in syntax highlighting, I believe Google Code Prettify is used so any related feature requests would have to be posted upstream. However, do note that highlighting is performed based on syntax, not markup, so the sample usage as you've posted in the question is unlikely to be adopted.
In the case of your answer where the default highlighting is potentially misleading, a possible compromise would be to disable syntax highlighting (it would however make your
grep --color example moot). Simply use
<pre> instead of indenting the code block, or precede the code block with
<!-- language: lang-none --> (see syntax highlight language hints).
If specific text colour is absolutely essential to the post, use a small screenshot.