I adore Terry Pratchett's books and recognise that translating them to film would alway to be a challenge - what can compare with the product of his writing and your own imagination? But Hogfather worked well and Going Postal wasn't too bad, considering it seemed rather low budget, especially in the special effects of creating the golems. But The Colour of Magic simply does not work.
Maybe Terry Pratchett is a great David Jason fan, or vice versa, but, as Rincewind, Jason is totally miscast. Rincewind is tall, gangly, rather despicably self-centred and cowardly. David Jason is, as always, David Jason - cuddly, slightly muddled and loveable. In no way a realistic Rincewind (I have to admit too that his Albert was, for me, the low point of The Hogfather - too much David Jason, not enough Terry Pratchett.)
And Sean Astin as Twoflower the tourist? Who on earth thought of that? What, he played a (passable) hobbit therefore he is indispensible in filming fantasy novels? Pleeease! Dreadfully weak in characterisation. And isn't Twoflower meant to be oriental?(Interesting Times, his return, ends up with terracotta warriors!) Or did the powers-that-be think that casting him as oriental would lay them open to cries of racism? Wussies!
I woud love to say that this enthralled me - but I can't. It simply left me unsatisfied and very disappointed.