I think Eco Complete is mined from some volcanic rocks, and ADA substrate is a magic manufactured mixture of clay, peat and whatever else. The one substrate that is dark, and you didn't mention was Soil Master Select Charcoal color or Turface. I believe aquariumplants.com packages it in red buckets called "
Aquariumplant's Substrate." It's cheap and will do the trick. Or if you have a
www.lesco.com or
http://www.oildri.com/agri/turforn.htm distributer (maker of this gravel) around your area, you can pick it up yourself.
Though a massive amount of carbon is an intriguing idea which I think would serve just fine as a substrate, it's more expensive then purchasing a commerical planted substrate.
-John N.