This, the very first of Pratchett's Discworld yarns, still holds up well today. His humor isn't quite as madcap as it is in later entires in the series, and there is an episodic structure to the storyline, but it remains the book you might just as well read first if you're going to read any of the Discworlds at all. It will give you a feel for how Pratchett matures as a humorist and storyteller over subsequent books.
The Color of Magic — which is, in Pratchett's world, "octarine," the eighth color of the spectrum — introduces us to Rincewind, the most inept wizard ever to flunk out of Unseen University. Rincewind doesn't seem quite the hopeless dolt he appears to be in Sourcery; he has his moments of self-assurance, particularly when he defies Death. But his story is still funny as he finds himself conscripted to play the role of tour guide to one Twoflowers, a gleefully idiotic tourist from far away in the Counterwieght Continent, who shows up in Ankh-Morpork one day accompanied by the Luggage (that creepy and temperamental living suitcase made of sentient wood). Twoflowers, who has an unhealthy eagerness to witness bar fights, starts throwing around massive amounts of gold like nobody's business, and Rincewind can't decide whether he really ought to stick around and save the poor clod from his own folly or take the money he's already been paid as tour guide and run. The decision is made for him, though, and the improbable duo soon find themselves responsible for burning down nearly the entire city as they flee into a host of adventures guided by the die rolls of the whimsical Gods.
It's interesting to see Pratchett play around with concepts that he would later develop more fully in later volumes. (Most amusingly, an introduction informs us that the Discworld series will contain "at least ten" books.) Death appears as a bit player here, and you get the idea that Pratchett already is formulating his nefarious plans for this character in future episodes. Unseen University is mentioned, but, of course, Unseen. And yet Ankh-Morpork already seems a well-thought-out setting, and Pratchett already has a solid talent for comedy writing, though at this early stage in his career the influence of Douglas Adams is evident. Also, unlike later books in the series which can easily be read as free-standing stories, The Color of Magic has a direct sequel, The Light Fantastic.
So pick up this debut entry if you haven't already done so. True, you can jump into the Discworld saga just about anywhere, but it makes sense, as the old song goes, to start at the very beginning...