The Colour of Magic is a very misleading title for this film. It is based not on the first Discworld book by Terry Pratchett but on the two first. The Light Fantastic is encompassed by the second part of the film which bares the same title. As such, the film varies from the book in that it meshes the two plots together... or rather, one plot, since as Pratchett says himself, the first book is just a series of funny stories.
Funny stories they are. Thieves, assassins, imaginary dragons, giant stone trolls, magic swords stuck in stones, treacherous and power hungry wizards, heroic barbarians. Epic plot to save the world included. We're getting ahead of ourselves, aren't we?
The Colour of Magic begins with the meeting of Rincewind and Twoflower. The first a failed wizard, expelled from Unseen University due to his inability to learn any spell. The latter is the first tourist, not knowing the value of his gold and a romantic optimist seeking adventure from his mundane insurance clerk job. The Luggage, a walking box with homicidal loyalty towards the tourist seems to be another character in this group, later joined by Cohen The Barbarian, the eighty year old retired Greatest Hero of the Discworld.
The film is very well made, much more interesting than the books it was based on. The characters are interesting and parodies of certain fantasy tropes. However, like with all parodies, it is hard to enjoy them if you don't know the references used. There are visual and audio jokes within and at least a few puns.
What else can I say? I'm too impressed to find much fault in it. I keep having flashes of the humorous parts and they were many of them, this is not to say that the film lacked cohesion. Far from it, using the plot of The Light Fantastic to create a plot throughout two parts proved a very good move, even though some scenes from the Colour of Magic couldn't be included in order to create new scenes with Trymon, the main villain of the film.
Lastly, I can say that to understand the film, you don't have to have read the book. I didn't read it before watching this and I still feel it's a film worth watching.