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Assuming we still got the same titles with no changes other than the box art, blue would be articuno, yellow would be zapdos, red would be moltres, and green would get mewtwo since that's what's left (given mew was a secret one of the devs snuck in) even though it only fits if it's shiny which wasn't even a thing until gen 2, maybe that would've caused them to make mewtwo green and the purple one would be the new shiny.
Otherwise it'd have been blue/yellow/red at initial release with the birds being version exclusive like legendaries today to promote trading and all 3 being needed to unlock mewtwo's chamber in cerulean cave, then we'd get purple version still based on the anime but ending with the first movie's events and after catching mewtwo trading over the birds unlocks mew as a roaming encounter like the beasts in gen 2.
