If you plan on having shrimp in your tank, think twice about the FloraMax. I've been told and have read a couple different threads on how it isn't the greatest thing to use in shrimp tanks. However, someone else may be able to give some other input on its effects with shrimp.
Either way, I thought it was pretty nice in my tank. It's about the same size and texture as the normal Flourite, so it mixed pretty nicely. With that being said, if you wanted a darker substrate, you could use black sand. I can't remember what it's called (Black Diamond Blasting Media?), but I know quite a few people that use it and have had good things to say about it.