why does coconut ice turn brown after a couple of days
Whenever you have a fresh fruity thing go brown, the word is "oxidation." 

There are a couple of ways to prevent oxidation in sorbet or other fruit ices. The first, and easiest, is to cook your mix before you freeze it. You'll lose some of that "fresh" taste, but your color will be more stable. 

The second is to add an acid...Acids love oxygen, and bind it away from your fruit (for a while)...This may not work in your case, since I imagine "Sorbex" is a version of "Fruit Fresh" which is a citric acid powder. Regardless, this will obviously effect the taste. 

Finally, you can grind up a chewable vitamin C tablet (or buy powdered Vitamin C) and mix that in.