Try the following (with the DLL in the AviSynth plugins folder):
converttoyuv32or
yuv(0.9,0.9,-100,100, 0,0,0) # 90% saturation and shifted UV channels
convertbacktorgb32
converttoyuv32DE
yuv(0,0,0,0, 255,240,200) # adjusts colours so that beige becomes white
convertbacktorgb32