I've been looking but I can't find an 'official' definition of infra and ultravision, only that they both let you see in the dark, and ultravision is better. I am assuming they use infrared and ultraviolet because of their names, and because infravision looks reddish whereas ultravision seems to emphasize the blues.

(Interesting, all ultravision is not the same, because although gnomes and dark elves both use ultravision, things look slightly different to the eyes of a dark elf than to the eyes of a gnome.)

I still say you can see in complete darkness with ultravision, though. Because, there is a place in Neriak where it is totally dark - in the hidden passage between Neriak Foreign Quarter and Neriak Commons. There are no light sources at all. I took a human there and he was completely blind, could not see a thing. Had to run and buy him a fire beetle eye from a vendor. But I could see fine with my gnomish ultravision.