Trail Renderer Fade- changes to color array have no effect

I have a moving sphere with a trail renderer applied to it, and I am trying to impart a fading effect on the trail. One previously suggested method was to change the alpha value of different members of the color array [0,1,2,34]. This sounds good in theory, but when I make changes to the color values, there is no observable effect. It seems that the color of the trail is governed by the material of my sphere object, and does not respond to changes to its array values. Can anyone help me here? Is there a smarter way to accomplish what I am trying to do?