Look forward to the pictures. You have piqued my curiosity as well. I have never heard that name, and can't see how it could have arisen. That said, half of all "common names" are made up on the spot by shops that know no better.
ah the infamous 'crossing rainbowfish' aka the royal rainbow and regal rainbow
this is a generic term given to any Melanotaenia trifasciata that has been crossed with another species of Melanotaenia.
If they're hybrids, that could account for the common name, although they are usually more subtle then that. Royal and Regal I've not seen either. I am well aware of how easily many rainbow species hybridise, and several "species" are hybrids, Melanotaenia marcii and Melanotaenia greetii are such and were doing the rounds here last year.