Hover the mouse over a field on the materials tab (alpha channel on this example)
and press i to insert a key.
Move a few frames ahead (arrow keys), change the value and press i again to insert a new key.
We now have a spline animating that field from one value to the other. It works just like normal animation.
A grey button means there are no keyframes. A yellow color means There is a keyframe on that frame, a green button means that the values have been animated, but there is not a keyframe on that specific frame.
An alternative to pressing the letter i is to right click on the value you want to animatate and select Insert Keyframe;