A common habit on the bug tracker is quoting text written by other people who comments on the issue, as for e-mails. 
I think it would be more clear if such text had a different color.
Toughs?

I don't see quote button at all.

My intention was not to introduce a quote button but have the server parse the messages, check for "line.startswith('>')" and assign a different color via CSS.

Sure, good idea.  Do you know if there is software out there doing that?

Trac - http://trac.edgewall.org/ticket/9220#comment:12
Rietveld - http://codereview.appspot.com/2442042/diff/7001/upload.py - expand collapsed comments and see how quoted text is handled there.

Can somebody propose a specific CSS that should be used? In particular, what should the colors be?

Being a fan of google mail, I personally like #500050 color, which I've also seen used else where in public forums and similars.

Not "line.startswith('>')", but "line.startswith('> ')". ">>>" used as Python prompt in code examples.