It seems Green Lanterns can switch rings easily as long as they acknowledge the emotion of the corps in question. Guy Gardner and Hal Jordan have already worn the red and yellow ring respectively, so it makes sense for them. But for me it’s a huge stretch for Kyle Rayner and John Stewart to be able to use the blue and indigo ring immediately.
Doesn’t make this scene any less cool though.
From – Green Lantern Vol. 4 #65