Again, you can't tell by the pic, but in the pic of the entire animal, it looks like the oral disk is more blue than the tips of the tentacles. That is very unusual. I hope they aren't starting to dye giganteas.
FWIW: I searched a couple wholesalers once or twice a month for over 5 years looking for a mertens carpet. I would look under every haddoni in the bins for spots. I would look under the giganteas trying to determine if those spots were magenta (mertens) or purple (gigantea). When I finally saw a merten's it was immediately obvious that it was so different from the other species of carpet, that looking under the disk for spots had been a total waste of time. It is not always so obvious in pictures, but when you see one in person, there is no doubt as to whether or it is a mertens. You don't even have to know what the specific differences are. It will just be totally different than any carpet you have ever seen.