Team-specific unlock
Color 5: Clear Episode Mode
Complete one team's Episode story and Color 5 unlocks for the four members of that Episode team. It is not a global unlock: finishing Fighting Avengers does not automatically hand the same slot to the other four launch teams.
The clean completion route is therefore to clear all five team Episodes. That also gives you the story and trophy progress you would otherwise have to return for later. Clearing your first Episode has a second, more important reward: it unlocks the hidden playable Champion.
Champion exception
A fresh Aug 11 player report says the Champion's fifth color appears after all five Episodes, but we have not found enough independent confirmations to promote that detail to a verified rule. Treat it as NEEDS_REVIEW; the team-based Color 5 rule for the 20 launch fighters is the part supported by multiple reports.