I have a copy of the most recent paper distinguishing all the species of the genus, that's how I learned to tell the difference. Most specimens coming into the U.S. today are misID'd. Could have been dorsal patterning on the abdomen, I wanted to be specific.
The coloration of these guys is unuseful tool for ID them.
You should check for palpal trochanter and if it's identical they should be the same species.
As for distinguish the diadema from variegatus as both species share the same "group" in Damon genus You should count the spines.
In diadema (as well as another species - D. brachialis, non-Tanzanian) they're are 2 on palpal trochanter, in other species of this group incl. D. variegatus - only one spine.