public void setActiveColor(final Color COLOR) {
        if (null == activeColor) {
            _activeColor = COLOR;
            fireTileEvent(REDRAW_EVENT);
        } else {
            if (!activeColor.isBound()) {
                activeColor.set(COLOR);
            }
        }
    }
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