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These colors are a reference to redshifting and blueshifting in astronomy. When objects travel TOWARDS you in 3D space they shift blue, when they travel AWAY from you they appear to shift red. For our legendaries, what they're symbolizing here is that the FUTURE always looms toward you, but the PAST always recedes away.
The story will probably involve time travel in some limited way, even if we don't actually see it and it's just told to us. The legendaries looking like bikes serves a double purpose, that being they travel (as in literally TRAVEL) through time. It is common for time machines in media to be literal cars or bikes or whatever that literally "travel" into the future/past.
