public ObjectProperty<Color> activeColorProperty() {
        if (null == activeColor) {
            activeColor = new ObjectPropertyBase<Color>(_activeColor) {
                @Override protected void invalidated() { fireTileEvent(REDRAW_EVENT); }
                @Override public Object getBean() { return Tile.this; }
                @Override public String getName() { return "activeColor"; }
            };
            _activeColor = null;
        }
        return activeColor;
    }
Defines the color that will be used to colorize the knob of
the radial tiles.

@param COLOR
src/main/java/eu/hansolo/tilesfx/Tile.java