function toColourspace (colourspace) {
  if (!is.string(colourspace)) {
    throw is.invalidParameterError('colourspace', 'string', colourspace);
  }
  this.options.colourspace = colourspace;
  return this;
}
Set the output colourspace.
By default output image will be web-friendly sRGB, with additional channels interpreted as alpha channels.

@example
// Output 16 bits per pixel RGB
await sharp(input)
 .toColourspace('rgb16')
 .toFile('16-bpp.png')

@param {string} [colourspace] - output colourspace e.g. `srgb`, `rgb`, `cmyk`, `lab`, `b-w` [...](https://github.com/libvips/libvips/blob/3c0bfdf74ce1dc37a6429bed47fa76f16e2cd70a/libvips/iofuncs/enumtypes.c#L777-L794)
@returns {Sharp}
@throws {Error} Invalid parameters