This is my "Out there" theory as I'm not 100% about it nor have a read anything to completely support this.
Maybe brown coral grows faster because its healthier? If you look at the majority of coral in the wild it appears to have more brown hues (possibly because of lighting) but growth rates in the ocean we can agree are usually better than our tanks. Maybe this is because the coral is packed with extra zooxanthellae supporting more cellular photosynthesis resulting in faster growth? Just my 2 cents, any advance SPS keepers please feel free to rip it apart
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