I'm using this Chrome Extension https://chrome.google.com/webstore/detail/user-css/okpjlejfhacmgjkmknjhadmkdbcldfcb
The part with [data-idmem="50d439c16f2647e533001ec7"]
allows to color member avatars. Edit with your own (check Trello source code to get IDs).
Create a black tag with the name "Done" to mark task as done visually.