getPhaseGradientColorMapMidColor() {
    const p = this._gradientMapTransitionPhase;
    const ps = p * p * (3.0 - 2.0 * p);
    const ips = 1.0 - ps;
    const fm = this._fromGradientColorMapMidColor;
    const tm = this._toGradientColorMapMidColor;
    return {
      r: fm.r * ips + tm.r * ps,
      g: fm.g * ips + tm.g * ps,
      b: fm.b * ips + tm.b * ps,
      a: fm.a * ips + tm.a * ps,
    };
  }
Gets the current Gradient Color Map mid color point accounting for phase.
@return     {cc.Color}     color   The color value.