Wizardry: Invisibility Spells
There's a surprising number of invisibility-related spells in Player's Handbook. I think almost all of them could belong both to the colleges of Light and Darkness and Illusion and Creation magic. Invisibility is your normal invisibility that gets dispelled if the subject attacks. Greater Invisibility does not have this drawback, but lasts a much shorter time. Mass Invisibility affects multiple creatures, but it also breaks if one of the subjects moves too far away from the others. Invisibility Purge negates invisibility around you, but is an indefinite spell, so it takes up your spell slot. Invisibility Sphere works slightly differently, creating a spherical effect around the recipient. See Invisibility lets you see through magical invisibility, and its improved version affects all kinds of invisibility (according to GURPS Powers, See Invisible that is part of a cosmic powers affects all forms of invisibility, so a +50% is fine).