mammillaria cowperae

mammillaria cowperae - dark reddish brown spined

yellow spined....

Both are beautiful.

M. cowperae or M. moelleriana?
I've seen many plants that look the same but with one or other of these names. I was told once that the yellow spined plants were cowperae, but I've also been told that when cowperae was first described it was of a plant with brown spines. Also I have seen plants called moelleriana which have yellwo flowers and some which have pink flowers. I have never seen a plant called cowperae with pink flowers though!!
Are they the same or different? and if so what is the difference?