Like for all such specific uses, I implemented Lua scripting...
With Lua you can color the chat of any avatar.
An example is given
in the manual for coloring name tags and mini-map dots (via the Lua pie menu), that could be extended to color the chat as well (using the onChatTextColoring() Lua callback).
There also could be more complex implementations via Lua, such as a custom text search floater, using the new context menu "Copy to Lua" feature to send the chat text to that floater...