code: public static Color darker (Color color, double fraction)
  	{
  		int red   = (int) Math.round (color.getRed()   * (1.0 - fraction));
  		int green = (int) Math.round (color.getGreen() * (1.0 - fraction));
  		int blue  = (int) Math.round (color.getBlue()  * (1.0 - fraction));
  
  		if (red   < 0) red   = 0; else if (red   > 255) red   = 255;
  		if (green < 0) green = 0; else if (green > 255) green = 255;
  		if (blue  < 0) blue  = 0; else if (blue  > 255) blue  = 255;
  
  		int alpha = color.getAlpha();
  
  		return new Color (red, green, blue, alpha);
  	}
docs: Make a color darker. (RGB color scheme)
  
  @param color     Color to make darker.
  @param fraction  Darkness fraction.
  @return          Darker color.