-- Native-pixel colour correction only; preserve geometry, alpha and all other art.
local sprite=app.open(app.params.source)
local image=sprite.cels[1].image
local mode=app.params.mode or 'portrait'
for pixel in image:pixels() do
 local c=pixel()
 local r,g,b,a=app.pixelColor.rgbaR(c),app.pixelColor.rgbaG(c),app.pixelColor.rgbaB(c),app.pixelColor.rgbaA(c)
 local y=pixel.y
 local in_hair=false
 if mode=='portrait' then
  in_hair=y<tonumber(app.params.limit or '122')
 else
  local local_y=y%64
  in_hair=local_y<tonumber(app.params.limit or '34')
 end
 -- Brown skin and gold trim fall outside this cool/neutral hair ramp.
 if in_hair and a>0 and r>=18 and r<160 and b>=r*0.45 and g>=r*0.64 and r>=b*0.92 then
  local lum=0.299*r+0.587*g+0.114*b
  local value=math.floor(lum*0.66)
  pixel(app.pixelColor.rgba(math.max(7,value-4),math.max(9,value-1),math.max(12,value+3),a))
 end
end
sprite:saveAs(app.params.output)
sprite:close()