White portals uniformly make up 4% of portals you encounter across all warp rides, at random. The only exception is if you're going through the ride for the first time on a save file and haven't yet been to Ultra Megalopolis yet; in that case the first attempt still puts white at 4% odds, but if you get a different color instead and have to go back, your second attempt will increase the frequency of white portals to 16%, then if necessary a third try will make it 32%, a fourth try 44%, and finally the fifth try gives up and forces white portals to account for 100% of all portals, ensuring that you no longer get to land anywhere else.
