My personal answer to the question is no. Inharrently nobody has a right to claim a color all to themselves, and in all reality the NPO can not stop alliances from being part of the red sphere, because they phisicaly can not remove nations from the sphere.
Nobody protests this or at least not a enough people from the red sphere, because in truth if enough nations became upset with the NPO imperial occupation of the sphere they would be over through. The NPO doesn't hold enough of a percentage of the red sphere to controle it, however given all that the NPO does for the red sphere via the red protection court, senate stability, and simply the economic advantages that exsist with having just one alliance in the sphere, red will not over turn the doctrine.
I think out side influance could shake things up for the sphere much like what Vox did with the senate seat as well as simply how large they became, but unless the NPO does something to ceriously anger the nations of red, nobody looses anything by them having sole controle, so while they have no right or ability to keep the sphere to themselves, I see nothing wrong with them acting like they do.