code: public static final double getGrey(final double r, final double g, final double b, final double redWeight, final double greenWeight, final double blueWeight){
  		return r*redWeight+g*greenWeight+b*blueWeight;
  	}
docs: Calculates a grey value from the specified RGB color using the specified
  weights for each R,G and B channel.
  <p>
  It is advised to use non-negative weights that sum up to 1.0 to get a
  grey value within the same value range as the specified color channels.
  <pre>
  grey = r*redWeight + g*greenWeight + b*blueWeight
  </pre>
  
  @param r red value of the color
  @param g green value of the color
  @param b blue value of the color
  @param redWeight weight for red channel
  @param greenWeight weight for green channel
  @param blueWeight weight for blue channel
  @return weighted grey value of the specified RGB color.