Hi,
this feature is coming from a plugin discourse-bbcode-color, the change to make it work is very simple, we only need to add bbcode-color in this list: https://github.com/discourse/discourse-chat/blob/main/app/models/chat_message.rb#L135
However I don’t want to pollute chat with plugin specifics so we will have to create a simple plugin API to allow plugins to insert themselves in the list.
I’m sorry this won’t be on top of the priorities, I will mark it as pr-welcome though. Let me know if someone starts working on it, I can offer guidance. I will also reclassify this as a feature and not a bug.
Thanks for the report.