function onStartColors(tweenProp) {
  if (this.valuesEnd[tweenProp] && !KEC[tweenProp]) {
    KEC[tweenProp] = (elem, a, b, v) => {
      // eslint-disable-next-line no-param-reassign
      elem.style[tweenProp] = colors(a, b, v);
    };
  }
}
Sets the property update function.
@param {string} tweenProp the property name