Unfortunately, there is none such feature implemented yet.
If it's only about a single color changing slower you could use an AnimationPlayer node, add an animation where the color of the button is changing to the color you want. Then connect the signal mouse entered of your Button and start the animation via code. The same for exiting the button. (Well, I know this is a lot to for such a thing, but...)
Would recommend to change the color of the button by animating the property self modulate in the inspector. You could also change the color of a TextureButton this way.