Exactly not all JDs get the purple colonization. The strain that TUIC is selling is from a specific catch location. They carry the gene for that specific color morph.
Yep, not all dempseys are the same dempsey. In fact there are 3 species of dempsey, Rocio gemmata, Rocio ocotal, and of course Rocio octofasciata. Each specific to different parts of the dempsey range(s) and of course each with slightly different color, shapes, or physical traits. The typical hobby JD from the FL or Asian fish farms is almost certainly a mix of any of the 3. why? simple, for decades it was just 1 species..
fun note. The electric blue dempsey. in 2000 Jeff Rapps commissioned & funded DNA workup on the species(extremely expensive back then). They in fact came back as pure dempsey(all were considered octofaciata at the time still) That was back then.. today who knows what they are probably mixed with. I know, I know, yeah but "why aren't they found in the wild?".. well, gotta know where to look
check this clip out from Cenote Cristalino, Yucatán Peninsula, Mexico. The cichlids are Rocio gemmata, the smaller growing ‘Jack Dempsey’ that is also found in Yucatán cenotes. Check out the nearly all electric blue stud swim by towards the end of the clip! wowzers!
link to video:
http://instagr.am/p/DCErCHnRd9B/
Also, your fish is young and on a dark substrate. So besides it blending in with its surroundings, being young it will color up as it ages.