We have used Minwax white wash pickling staIn on wood before with very good results. White sure gives the best 8bit shading. We have not tried any other colored stains.
We use a light coat of clear paint to seal the engraving afterwards so it does not fade. Too much will dull the laser engraving. We also use fine sandpaper across the grains before engraving to reduce the raw look with bare wood. We found Poplar or Birch with minimal grains to be best for laser engraving.