code: func (f *Fpdf) GetFillSpotColor() (name string, c, m, y, k byte) {
  	return f.returnSpotColor(f.color.fill)
  }
docs: // GetFillSpotColor returns the most recently used spot color information for
  // fill output. This will not be the current fill color if some other color
  // type such as RGB is active. If no fill spot color has been set, zero values
  // are returned.