err... um what?
Edit - Here's the 4* Hulk covers given out recently from this months sneak peak:
Claws for Concern - All Wolverines (4/3/18)Progression Rewards:4-Star: The Hulk (Totally Awesome) - Blue3-Star: Wolverine (Patch) - Red, Green2-Star: Ares - Red1-Star: Venom (Dark Avengers) - BlackPlacement Rewards:4-Star: The Hulk (The Main Event) - Green, Yellow3-Star: Mystique - Blue, Purple, Black2-Star: Storm (Classic) - Blue, Green, YellowAlliance Placement Rewards:3-Star: Mystique - Blue2-Star: Storm (Classic) - Blue, GreenIdentity Theft - Mystique (4/5/18) Progression Rewards:4-Star: The Hulk (The Main Event) - Purple3-Star: Mystique - Purple, Black2-Star: Wolverine (Astonishing X-Men) - Red1-Star: Spider-Man (Original) - PurplePlacement Rewards: 4-Star: Jubilee - Purple, Yellow3-Star: Rocket & Groot (Most Wanted) - Green, Yellow, Blue2-Star: Thor (Marvel NOW!) - Red, Green, YellowAlliance Placement Rewards: 3-Star: Rocket & Groot (Most Wanted) - Green2-Star: Thor (Marvel NOW!) - Red, Green
Cover colors in PVP prog and placement go through a predicted rotation, though with 60+ 4* the cover rotation is slow. Same with DPD vaults I believe the color of the 4* offered is predetermined on a rotation basis.
Other vaults it's random and also randomly when out of tokens, but all colors for a character have the same chance of appearing. If you're only seeing one, it's a bad RNG time for you. See my 0/0/5 5* Doc Ok for an example of RNG evil!
Also from the earlier sneak peak.
Ragin' Cajun - Gambit (Modern) (3/29/18) *New Update*Progression Rewards:4-Star: Gwenpool - Black3-Star: Gambit (Modern) - Red, Purple2-Star: Storm (Classic) - Green1-Star: Venom (Dark Avengers) - BlackPlacement Rewards:4-Star: The Hulk (Totally Awesome) - Green, Black3-Star: Wolverine (Patch) - Yellow, Red, Green2-Star: Thor (Marvel NOW!) - Yellow, Red, GreenAlliance Placement Rewards:3-Star: Wolverine (Patch) - Yellow2-Star: Thor (Marvel NOW!) - Yellow, Red