The Colour of Magic is the weakest of the 3 Sky telemovies - and it's the only one I didn't buy on DVD. I think David Jason was woefully miscast as Rincewind (he was a decent Albert, however), but I can understand why they did it from a "star name" point of view. Some of the performances were good (e.g. Tim Curry) and the way they managed to pull together some of the disparate bits of the two books together worked, but as a whole, it's just "ok". Whereas Hogfather is "good" and Going Postal is an exciting retelling of the novel (even with some plot details changed).
I guess a lot of where I come from is having done more than 15 of the stage plays. Briggs does a good job of adapting the novels for the stage, but many a subplot is excised (with good reason). He tends to stay much truer to the dialogue of the books than the film versions do (usually lifting Terry's writing verbatim), which is wonderful, but I can also understand why screenwriters feel the need to adapt the words to something more concise, as they usually have less time to play with. (That being said, I was still amazed at how Going Postal managed to use quite a lot of direct dialogue from the book, albeit sometimes in entirely different scenes...)
But... anyway, back to The Amazing Maurice. If you are happy to accept that it's a film designed for a 6-10 year old audience (and their parents), and it's going up against films like Minions or Puss in Boots or whatever the latest Disney one is (Strange World, I guess, although that seems to have flopped - but it's still a very good film!), then what they've managed to do to appeal to that audience *and still* keep it pretty faithful to Terry's story is quite, if you'll excuse the pun, amazing.