Also you can use GreenSock Konva Plugin for tweens.
GreenSock tweens are more powerfull than Konva's tween.
Also demo demonstrate tweening fillLinearGradientColorStops
property with usual GreenSock usage.
{% iframe /downloads/code/tweens/Complex_Tweening.html %}
{% include_code Konva Complex Tweening Demo tweens/Complex_Tweening.html %}