From my knowledge these are the colours available to use in the chat (unless they somehow have extra colours from a plugin or something).
Black just wouldn't look very nice in the chat.
Dark Blue I'm not very fond of either (I also don't think it looks good in the Minecraft chat from what I remember).
Dark Green is global chat.
Dark Aqua is [Veteran].
Dark Red is used for plugin messages like LWC etc.
Purple is used in /msg.
Dark Yellow I believe is [VIP] but it looks different on the website to in-game.
Grey and Dark Grey aren't very appealing.
Indigo is mining world chat.
Bright Green is [Prefect].
Aqua is [Supporter].
Red is [Admin].
Pink is [Mod].
Yellow is local chat.
White is [Member].
Now that I've had a long thought about this the only colours I see that could be suitable would be dark red or dark yellow. That's just my opinion though.