Without a scope to see the flukes there is no assurance that the problem is flukes. Normally, for a shotgun treatment to treat for unknown parasites we would recommend the use of Proform C, three treatments followed by the Prazi or Fluke M, but the label instructions on Proform C states not to be used with salt. John of Hanover Koi has used both salt and Proform C and says that it is ok, but being off label means do at your own risk. Most of the parasites became salt resistant is the reason for going against salt, but that was at 0.3% salt, and I believe most are still susceptible to 0.6%. Do you have a salt meter? Trying to take the salt to 0.6% without knowing the actual pond volume could be dangerous as many ponds due to sloping sides, irregular shape, uneven bottom are not as large as calculated and sometimes by as much as half, meaning the dosage could be double what you are trying for.
The only reason I have addressed the treatment above is because you mention the fish becoming lethargic. The change in color could be due to unstable color genetics. The Chagoi (brown), Soragoi (gray), and the Ochiba (brown and gray) are very closely related and the Ochiba in particular will change color and pattern if not real stable, and that looks like what is happening with the color. Not something to worry about.
Zone 7 A/B
Keep your words sweet. You never know when you may have to eat them.
Richard