Handwavy (early and my morning caffeine hasn't kicked in) but if you're trying to uniformly desaturate all colors what I'd do would be to walk the pallet instead (not swapping the image to true color of course):

• Get the total number of pallet entries
• For each index 0..$num_colors
  • Fetch the rgb for the index
  • colorDeallocate that index
  • Compute your new desaturated color and then colorAllocate it back (which should (?) put it back in the recently freed index)

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