God as the Greatest Conceivable Being

It has been claimed that "God is the greatest conceivable being".  I usually interpret this to be a specific claim about the specific god held to exist by a specific claimant, but I note that it also applies more generally to the god of any WLC-affiliated theist, because WLC makes a similar claim in his variation on Plantinga's version of the ontological argument.

I put it to you though that I can conceive of a being that is greater than the god of any specific theist.  This being, for the sake of the argument, is the Grand Pixie, which has not only all the characteristics of the god of the theist, but also purpleness.  In fact, the Grand Pixie is purpleness so the Grand Pixie has "being purpleness" as one of his characteristics.

Now, I claim that "being purpleness" is a feature that improves on anything that has all the other characteristics, even if "being purpleness" is entirely neutral, because it's one more characteristic thus contributing to a greater grade of numerical greatness.

An argument that might be raised against my argument here is that no-one believes in the Grand Pixie, but I respond that the number of believers is not only irrelevant but also damaging to the "my god is the greatest conceivable" argument, since a conceivably greater god than anyone's god would be one that is believed in more people - since I don't believe in any god, then a conceivably greater one is the one that I too would believe in, being one more believer.

Another is that I myself am the person making claims about the Grand Pixie and I can't refer to someone back in history who made the claims.  True, but this is just context.  The same applies if a theist goes back to the original claimant with respect to the maximal conceivable greatness of her god, even if we don't know who that original claimant was.  The only way out of this regression is to arrive at the maximally great god and have it tell someone that it is maximally great, but this is a sort of thing that non-maximally great things are also capable of (demons for example, in the worldview of some theists, and the figments of insane people in the worldview of some atheists).  The point here being that the claim to maximal greatness made by a human on the part of her god is precisely that, a claim, and nothing more.

I can make a counterclaim that there is a conceivably greater being than anyone's god, the Grand Pixie where Grand Pixie =your god + "being purpleness"/some other feature absent from your god, say "grooviness" or "being paisleyness".  The theist can try to address that, by claiming that her god actually does have "being purpleness", "grooviness", "being paisleyness " or anything else that I might come up with, but then the theist’s conception of god is revealed to be ad hoc.

While some might accuse me here of being insulting to their conception of god, and there might be some truth to that although my intent is more towards light-heartedness (and I know that some people are insulted by people not talking about their god with anything other than utter seriousness), the central point remains.  No matter how great you make your god, someone else can come along, add even a neutral feature and conceive of something greater.

So how can anyone truly claim that their god is the greatest conceivable being?