Sometimes it can be pretty difficult to determine the narrative technique of a novel. If you've had difficulty with this in the past, though, don't worry about it with The Color of Magic—the hardest part of determining this novel's narrative technique is spelling omniscient. That can get pretty tricky without spellchecker.
The novel makes the narrative technique very clear, very early in the novel:
All eyes in the room were watching the stranger—except for a pair belonging to Rincewind the wizard, who was sitting in the darkest corner nursing a mug of very small beer.
He was watching the Luggage.
Watch Rincewind. (1.4.21-23)
The narrator is clearly outside the story looking in. He's telling us what's going on, without being personally involved in any way—there's nary an I in sight.
With the third-person part established, we now have to determine if it's a omniscient or limited-omniscient narrator. Since the narrator can leap into any character's point of view—from Rincewind to Twoflower, the Patrician to the Arch-astronomer of Krull, and even the Great A'Tuin (1.Prologue.4)—we know he's not "limited" to just one or two characters. No limits equal a pure omniscient narrator.
Put them together, and what have you got? A third-person omniscient narrator.