I'm wondering if there's any way, even if via advice or re-definition, to customize the evil ex prompt. Sometimes I invoke it instead of
M-: to evaluate an expression, so I was thinking of showing a propertized string there to make it really stand out.
No, there isn't a customizable for that (see the
read-from-minibuffer call in
evil-ex). Feel free to open a PR if it bothers you greatly that the colon doesn't stand out, otherwise redefining the command (after it has been loaded) will suffice.