code: function tint (rgb) {
    const colour = color(rgb);
    this.options.tintA = colour.a();
    this.options.tintB = colour.b();
    return this;
  }
docs: Tint the image using the provided chroma while preserving the image luminance.
  An alpha channel may be present and will be unchanged by the operation.
  
  @param {String|Object} rgb - parsed by the [color](https://www.npmjs.org/package/color) module to extract chroma values.
  @returns {Sharp}
  @throws {Error} Invalid parameter