It turns out Mario isn’t the only playable brother in Super Mario Bros 35. There’s an additional playable character: Luigi. However, you can’t immediately go green. As both shared and pointed out by Combo on Twitter, you need to essentially prove yourself before you can play as Super Mario Bros 35’s Luigi. [Thanks, CombotronRobot and NintendoLife!]
Combo shared both video proof and details about how to access this alternate avatar in-game. In Super Mario Bros 35, playing long enough to hit and pass player level 100 then adds a star next to your name. (So you would see a star, then a number.) Once you’ve reached that milestone, you can hold down the L trigger when heading into a match to play as Luigi instead of Mario.
Here’s Combo’s video footage of the match. As noted, this player does have a star next to their level and was holding L until the match began.
— Combo @ Open Commissions (@CombotronRobot) October 20, 2020
Nintendo didn’t mention anything about Luigi being playable in Super Mario Bros 35 ahead of or at launch. Also, as someone who has not yet passed player level 100, I can confirmed I can’t trigger the playable Luigi with the trick.