Actually most corals, at least stony corals, don't eat algae or phytoplankton. They eat animals, or zooplankton. The idea behind feeding phytoplankton is that it helps develop a population of small animals in your tank, which then reproduce and get eaten by corals, usually in their larval stages when they're just moving around in currents. DTs is the best source of phytoplankton I've seen.

I would not supplement iodine at all; there's no proof that corals need it, and unless you can test for it in your water, you may already have elevated levels over seawater. Most aquariums do, just by the salt mixes. I don't know what chromaplex is, but it sounds like something you'd get for a camera.

Most people that have very colorful corals have very stable high alkalinity and calcium levels and vry strong intense lighting. The zooxanthellae in the corals are not responsible for the color, they're brown. Some people have written that the colorful pigments corals have may be somewhat in response to very high light levels and may be protective. It's definitely true that shallow water corals tend to be much more colorful than deeper water ones, where much less light and a more limited spectrum reaches them.