code: function(fillColor) {
              var fillColorHex = this.colorUtils.hexFromColor(fillColor),
                  luminanceThreshold = 128,
                  darkColor = 'black',
                  lightColor = 'white',
                  fillLuminance = this.colorUtils.getLuminance(fillColorHex);
  
              return (fillLuminance < luminanceThreshold) ? lightColor : darkColor;
          }
docs: use the value to determine the fill color, then use that color's luminance determine if a light or dark font color should be used