> I think having Emacs using red for error message in the echo area etc would look pretty odd? Anybody got an opinion here?

I don't think it looks odd, it just makes errors more conspicuous which is not a bad thing:

(run-with-idle-timer

0 nil

(lambda ()

   (message (propertize "Error in post-command-hook (my-test-post-command): (args-out-of-range 0 1)"

                        'face 'error))))

But it can have its own face (error-message) to use by error and signal, so people can customize it.