Rummynose Tetra is Yellowing

[DannyC]

Back around the end of January, I think it was, I got some rummynose tetras and one of them never really colored up. I quarantined them all together for a month and used the medicine regimen recommended by the Co-op. Of the 23 tetras I got, one never colored up red on its head. Having gone through quarantine with no visual issues and them receiving the meds i gave them I figured that maybe it was just a genetic defect since all the rest colored up and they all seemed healthy including the one that didn't color up. Fast forward until 2 days ago and the same one has still never colored up red on its head, but it has now begun to yellow as you can see in the picture I provided below. Does anyone know what could be causing this? Nothing looks wrong on its body other than the color issues so if it is something I would imagine it is internal. Tank temp is 82f, Ammonia is 0ppm, Nitrite is 0ppm, Nitrates are <~40ppm, and pH is 6.6. thanks for any insight!

[Colu]

Rummy nose that don't develope the red nose or have it fade can be a sign of poor health as he's eating and behaviour normal nothing in particular is jumping out at me  notice any rapid breathing hanging near the surface is he eating ok 

[DannyC]

It is acting completely normal and eating normally too. It's just the sudden color change that has me worried. It went from normal silver like the rest to a dull yellow in a day or less perhaps.

[Colu]

The colour change can also be caused by a parastic infection I would do a course of paracleanse in food