I do remember seeing some of Pratchetts books in the connect store about two months ago. I do not remember if "the colour of magic" was among them. The only thing I remember is that there were quite a few [understand MOST] of his books missing.
If you want to read *some* book from Mr. Pratchett I very much recommand "Small goods". I have read all his books and the "Small goods" is one of the best. I consider it to be the best from among the Connect store. Of course Your Mileage Can Wary (TM) ;-)
see:
http://en.wikipedia.org/wiki/Pratchett
http://en.wikipedia.org/wiki/Discworld