I'd really not recommend The Colour of Magic to people. Like, really really. I'd physically hide their copy from them if that wasn't wrong!
I'd recommend starting with one of Mort, Guards Guards or Wyrd Sisters to someone who really likes reading things in order, or anything after Small Gods to those who don't mind. I remember starting with Feet of Clay and falling in love. I think all of his books work well as a stand-alone. I remember being utterly nonplussed by The Colour of Magic. It's a very different book, and imo a very inferior book. To me, it'd be like handing over a copy of The Book of Lost Tales to someone who expressed an interest in Tolkien.
Some time back, I chanced to find a version of this chart: Terry Pratchett’s Discworld - Reading Order; and having read two or three of his Discworld novels, in no particular order, previously, I did find it helpful. But that's if we're talking about reading only Pratchett's Discworld stories; if we're talking about reading Pratchett in general, I think I would recommend starting with The Dark Side of the Sun (1975) and Strata (1981), inasmuch as they contain thematic precursors to many of the elements seen in the Discworld stories and The Long dot dot dot books, co-written with Stephen Baxter. Just to get a little bit of grounding.