sick boesmani rainbow

Hi, I have 2 Boesmani Rainbows in my community tank (with guppies,
mollies, platties and a gourami). I have had the Rainbows for about 4
months, and they seemed to have settled in fine, they eat and play and
have fun. I think they are adults (about 2"), not sure what gender,
they look identical. About 2 weeks ago, one of them suddenly had
turned dark purple brown to one side, from its head back to just past
its gills. It seemed to be happy enough, so I thought it might have
been a particular colour feature, like something to do with mating or
some other reaction to the environment. Now almost all of the brown is
gone, but it seems to be unhappy, motionless, staring into a corner,
not eating etc. It occasionally has a swim around, but I think it is
ill, and it could well have to do with the unexplained colouring. The
other rainbow seems just fine and unaffected. Anyone any idea what
might be going on here? Thanks!