It's obvious that they were the cycles used by the cops (blue), but in the scene where the disturbed Zipper shows John the new blue-colored Electra Glide he bought with the money he stole (shortly before being shot) I thought that might have some significance, also... don't know what, of course......
shareI think the Electra Glide motorcycle Zipper got near the end of the movie symbolized all that he hoped for in life. It was unobtainable by him through legitimate means, as he was not willing to work and save for it. He could only obtain it through the theft of the money, and that act would cost him everything he had--his job and his pension. So by taking the shortcut to what he thought would make him happy ultimately cost him his life.
Nobody gets to be a cowboy forever.