A scythe, midnight black robes, a shiftless skeletal smile, a voice composed in deathly ALL caps, and tasked with ferrying the dearly (and not-so-dearly) departed to the afterlife. Yep, this guy can only be Death. Discworld's Death may borrow his shtick and fashion tips from classic representations of the character, looking at you The Seventh Seal, but as a character, this guy's a Pratchett one-of-a-kind.
The Color of Magic marks the first appearance of Death, who has been featured in almost every Discworld novel since, from cameos (like in Equal Rites) to being a main character (like in Hogfather). He even appears in Pratchett's non-Discworld novels Good Omens? and Johnny and the Dead. Although a significant character within the Discworld series, Death only makes a few choice appearances in this first Discworld novel in an attempt to keep his "appointment" with Rincewind, who finds some mighty convenient excuses to postpone the inevitable.
Death can seem a bit crueler and less sympathetic here than he does in the later novels, too. In those other novels, Death is a lover of cats and curry, is fascinated by humanity, and is a dedicated family man. In this novel, however, Death takes the life of a fish salesman out of pure irritability, though "he [does]n't take much pride in it" (1.18.20). He also decides to make Rincewind's bounce from the mortal coil his new hobby because the wizard irks him something awful (2.13.2).
While it might seem like Death is a very different character than his later incarnations, we have a couple theories that might resolve the issue:
It's hard to pin down a true antagonist in The Color of Magic since the novel is really four distinct stories. But Death makes an interesting candidate for the position thanks to his relationship with Rincewind.
Rincewind is the novel's protagonist, and his main goal in life is to extend that life for as long as possible. Preferably in luxury and comfort, but he'll take what he can get. Death's goal, on the other hand, is to end Rincewind's life because it's only proper to die when everyone else expects you to. In one telling scene, we see Rincewind caught between a pack of ravenous wolves, a more-than-likely-poisonous snake, and a precarious fall from a tree, yet he defiantly tells Death that he can hang around all day if it means he can go on living (2.3.2-7).
That's basically the relationship between the protagonist and antagonist: One wants something and the other opposes him. Similarly, we can say this is the dynamic between Rincewind and Death.
But let's not forget that Death comes to a different conclusion by the novel's end. As he tells Fate:
I DID INDEED CHASE THEM MIGHT, ONCE, he said, BUT AT LAST THE THOUGHT CAME TO ME THAT SOONER OR LATER ALL MEN MUST DIE. EVERYTHING DIES IN THE END. I CAN BE ROBBED BUT NEVER DENIED, I TOLD MYSELF, WHY WORRY? (4.14.97)
Clearly, if Death was the antagonist for stories one to three, he is no longer antagonizing Rincewind by the fourth tale, and so it can be difficult to argue he is the antagonist for the entire novel.
Further evidence against Death as the antagonist is that no other character has the same relationship with Death that Rincewind does. Twoflower doesn't worry about dying because he's too busy living it up, Hrun's battle to stay alive is more instinctual than intellectual, and even the dead-to-be Greicha doesn't oppose Death. He merely postpones the trip to make sure everything works out in his Kingdom. And because of all this, we'll let you decide whether Death can ultimately be considered this book's true antagonist. Go wild.