WheelViewSample/src/main/java/com/lukedeighton/wheelsample/MainActivity.java
private int getContrastColor(Map.Entry<String, Integer> entry) {
        String colorName = MaterialColor.getColorName(entry);
        return MaterialColor.getContrastColor(colorName);
    }
private int getContrastColor(Map.Entry<String, Integer> entry) {
        String colorName = MaterialColor.getColorName(entry);
        return MaterialColor.getContrastColor(colorName);
    }
get the materials darker contrast