How to turn a PROTAGONIST blue+3 card into purple

Right off the bat, keep in mind Nalu's Blessing can be used as a "joker" card, that will work for any blue protagonists in order to upgrade them to purple.
Since 4.6 Update, all the protagonists you need can be easily obtained from the designated section in the "Events" section, and it will already be blue rank.

This list tells you which purple cards you need as a "feeder" card to evolve a blue+3 protagonist card to purple.  If you obtain one of these feeder cards as a drop or from a Hero summon, think very carefully before you decompose it!

(Please see this post for details of how to get the "feeder" cards listed below.)
Orange colored names can become orange cards too.

MT: Vem'tharak (Dark Entrance) or General (Dark Dungeons)
Barrel Bull: Vem'tharak (Dark Entrance) or General (Dark Dungeons)
President: Warchief Rehk (Dark Dungeons) or Continuum Lord (Lava Lake)
Tyran: Warchief Rehk (Dark Dungeons) or Continuum Lord (Lava Lake)
Bald Man: Warchief Rehk (Dark Dungeons) or Continuum Lord (Lava Lake)
Lady: Emperor Thoraim (Dark Crag)
Qriest: Princess Rustbeard (Dark Crag)
Miss: Death Observer (Dead Cavern)
Druid: Iranicus (Cathedral)
Female Druid: Iranicus (Cathedral)
Thief: The Mayor (Seturmoen)
Rogue: The Mayor (Seturmoen)
Hunter: General Surlysmith (Dark Crag)
Beauty: Inspector Gugemar (Dusk Town)
Fire Captain: Ambassador (Dark Crag)
Mage: Grandmaster Workid (Phoenix Valley)
Warlock: Remildare (Seturmoen) or Echo Hunter (The Dark Hold)
Blood Elf Mage: Ghalim (Psychic)
Brick: Chaz Rimewhisper (Psychic)
Shaman: Terror Lord (Seturmoen) or Gomula (Slave Stalls)
Soccer Team members (Howard, Thomas, Kevin, Neil, Issac, Gavin): Emperor Thoraim (Dark Crag)
Fire Brigade members (Fire Member A, B, C, D): Ambassador (Dark Crag)
Comrade: Remildare (Seturmoen)



The Four Shadows:

Allure: Calcula (The Energy Fortress)
Envy: Illusia (Psychic)
Lust: Chief Kaj (Broken Fortress)
Shame: Grandmaster Workid (Phoenix Valley)