[Brilliant™]

I've been seeing a couple of members using emojis and colorful Unicode characters in their usernames, wanted to know if that is allowed.

[Mr Kewl]

They are not disallowed unless you use to abuse them. Also I wouldn't recommend using them as if you logout and can't get your username for logging in(owait, you can login with email.)
So yeah, they're not disallowed.