On a whim, I clicked through to the Pixiv link on one of ni-iro's images, and the artist was credited as ISA on Pixiv. Curious as to whether this was just a Pixiv identity vs. previous identity problem, I went to the wiki and looked up their web page. It looks like the web page's name is "NI-IRO-丹色-", which is what's prominently displayed at the top, but if you scroll down to the bottom, it says the web page owner is ISA ("管理人 : ISA").

I would think the proper name for this tag would be isa ... except there's already a different isa artist tag. Maybe isa_(ni-iro) would be best.

This artist's pixiv login is ni-iro as seen in the url for the images but if the artist actually goes by isa then isa_(ni-iro) would probably be the best replacement, yes.

Aliased to isa_(ni-iro).