LivingCorpse said:
It's the title of the movie.
https://en.wikipedia.org/wiki/Ultraman:_The_Next
So character name "Ultraman The Next", movie title "Ultraman: The Next". The movie has ":" in the title.
Looking at it a bit more closely, apparently that's just the name in English. If you look at the movie's manga adaptation, it's "ウルトラマンTHE NEXT", no ":" present, and according to Japanese Wikipedia, the title with no ":" was the original title until they changed it to just be ULTRAMAN, so the manga's use of ULTRAMAN THE NEXT in tandem with that being the character's name suggests the ":"-less version is the correct one.