Most weddings on the island are from 4 - 4:30 PM, followed by photos until about 6. You'll get some nice pops of the blues & greens from the water, and maybe some nice golden lighting, but a colorful pink / red / orange sunset you might be thinking of isn't very common on the island. Checking out some of the photographer's blogs might give you a sense of what to expect.
You could always drive across the street through the end of Reef Village villas if you wanted to be facing west, less than 5 minutes by golfcart.