Every base player in FUT is Bronze, Silver, or Gold, and has a Common or Rare designation. CONMEBOL players are unique in being represented as a ""Special"" item design, but behave the same as Bronze, Silver, or Gold players.
The only qualifying criteria is that your team must be entirely composed of silver players. This can include Libertadores cards rated 65-74 and special cards like Team of the Week providing they fall within that ratings window.