The ideal solution to this would be to use a <span class="mod-note"> HTML tag around mod text, so that users could set their browser to make it whatever color they desire.
That being said, the choice of purple was somewhat arbitrary; I just picked it because I had seen it used that way on another forum and liked it. If it's decided that it has to be changed, though, it needs to be limited to one of the named HTML colors, because I doubt all of the moderators are going to want to memorize a hex triplet. Of the options presented above, I'm partial to teal for personal reasons, although I wonder how much contrast it would have with the light green post backgrounds. Red would be a poor choice for that reason, since red-green colorblindness is the most common form.
This may be a non-issue altogether, though. I can't think of many situations in which it would be unsafe to simply assume that a moderator's post is purple and act accordingly. It's more of a convenience to make clear in some situations that a moderator is, in fact, exercising their moderation authority.
