I think you might be getting confused with the term 'superfree', so to clarify
Green - all veg are syn free
Red - all veg except potatoes, parsnips, sweetcorn and peas are syn free
Extra Easy - all veg are syn free but the plan says that one third of your meal should be made up of superfree foods and that means only those veg that are free on a red day.
It doesn't mean that you can't have those four things mentioned above as they are still free, only that they don't dount toward your superfree requirement