Yes, new filament pushes the old out. Some filaments purge quite cleanly with quite small amount of new filament, some colors might require a quite long purge, like when printing black and white colors, otherwise the colors "bleed" on each other, usually the black taints the white. And when printing with solubles, I've noticed that in order to get all of the soluble material out, I need to purge something like 80mm+ of the new material, because the old and new material somehow mix in the nozzle and if there's still some of the soluble material, the resulting print will be weak on that spot.