OK, folks, I need your opinions. I've been doing some reading, and it seems that Sumatran soil is either reddish or dark gray/black.
@Circus pointed me to zoo med reptisand, which is made of inert quartz that won't leek iron into the water, and it's available in both colors. The red is more rusty colored; the black is more of a variegated dark gray. They both look very natural, and I like them both.
This will be a shallow tank, quite densely planted, with blue rasboras, kuhli loaches, dwarf rasboras, and imbellis bettas, so lots of greens and maroons in the plants, reds and blues with the fish. I am slightly inclined toward red just because I've never done a red substrate before, but I really like dark substrates generally.
What would look better? Red or black? Thoughts?