Articuno, Zapdos, and Moltres are not the mascot legendaries of the games. In fact, there are no mascot legendaries. The only mascots are Charizard (red) Venusaur (green), Blastoise (blue), and Pikachu (yellow).
Also, a bit of trivia for you: Pokémon Blue existed in Japan before it was released internationally. It was an enhanced version of the two games, with nothing new but different sprites and slightly different encounters.
The three legendary birds are based on seasons, with Articuno being winter, Moltres being spring, and Zapdos being summer. During autumn, these birds rest:
Moltres brings warmth in the coming of Spring
Zapdos brings the Summer storms, powerful and strong
Articuno rises with the harsh cold and a howling blizzard
In Autumn these three rest.
Zapdos ceases his storms as he finds his nest.
It then grows steadily colder as Articuno begins to wake.
This transition is called Fall, as the weather becomes colder and the
trees begin to drop their leaves.
Articuno’s awakening can be told by the first snowfall.
Sauce (thank you again, Assault!)
Edit: I also feel kinda obligated to mention that the Japanese word for blue can also mean green, however I don’t think this has anything to do with this.