The Color of Magic is very good, certainly, but many of Pratchett's books are better. For example, Monstrous Regiment and Wyrd Sisters are much more representative of the Discworld. I personally think that everyone should start off reading Guards! Guards! (the first novel about the Watch).
I've seen this sort of thing in other cases, too. If you want to play 'hardcore' board games, you try out Settlers of Catan first, despite the fact that every single Catan-like game published in the seventeen years after Catan has greatly improved upon it. It doesn't make sense, but everybody starts there so you start there too.
What book do you think new readers should read first? Tell me in the comments!