code: public double getGrey(final double redWeight, final double greenWeight, final double blueWeight){
  		return ColorPixel.getGrey(r_asDouble(),g_asDouble(),b_asDouble(), redWeight, greenWeight, blueWeight);
  	}
docs: Calculates the grey value of this pixel using specified weights.
  @param redWeight weight for red channel
  @param greenWeight weight for green channel
  @param blueWeight weight for blue channel
  @return grey value of pixel for specified weights
  @throws ArrayIndexOutOfBoundsException if this Pixel's index is not in
  range of the ColorImg's data array.
  @see #getLuminance()
  @see #getGrey(double r, double g, double b, double rW, double gW, double bW)