This brilliant colour is caused by IIV-31 (Isopod Iridescent Virus). I've been checking hundreds of pill bugs over several years for this unusual phenomenon and my persistence finally paid off!
Sadly, this total colour change is usually the later stage of the virus; once a pill bug is this bright it generally doesn't have much longer to live.
Found with warpaint shiners, fatlipsminnows, central stonerollers, whitetail shiners, and river chubs
I made a separate observation for the Scarlet Shiners
Over a River Chub mound. Only other species present were a few small central stonerollers.