I need to make a correction, uca was split up a ton in recent years, goes to show how out dated info on these guys are lol...
Paraleptuca crassipes is the best guess I have for the species. Which would explain why red/blue variants are sold. They typically live in mangroves. A common name for them is thick legged fiddler. I'd lean more towards full saltwater than brackish.
The species we're more familiar with, minuca mixas, is distributed all along the eastern coast of the US so it isn't specifically all brackish either. I'm not actually sure if brackish is a requirement or a symptom of going inbetween salt and freshwater since a lot of them are sold for freshwater hobbyist and temporary feeders.
It is definitely odd how they could ever mix up porcelain and fiddler crabs, I don't know what seller you're using but I'd def be careful with them.
I think its also paraleptuca crassipes. Color variations are all over the place with fiddlers.
https://www.fiddlercrab.info/u_crassipes.html