Each fish has its own genetic even they were from same parent, same breed batch, I saw you have a gourami in same tank. Does that Gourami giving your jag hard time? Stressing and not a alpha leader in your tank can lead to dull coloration, although Jag show their best color/marking around 6+ inches. Like I said in previous post your Jag look like a female jag so female won't have bold color or marking compare to a male jag.