I don't have any direct experience, but I've heard the "black" S. alatas grow much more slowly than others. *If true, one possible explanation is that all that pigment captures light that would otherwise support photosynthesis. *Lots of plants use red pigments to absorb excess light and protect chlorophyll. *That's why many turn red in strong light and why many red clones stay green in lesser light. *But the red pigments don't photosynthesize.