How to start a Time Node? my goal:

This is my first compositing attempt in Blender, my goal is to create an animation shown in the render window, the animation is a simple shift from one colour to another within about 100 frames. 

what I did and thought:

To achieve this, I put a composite node for displaying the rendering, I took two RGB-nodes, each with a different colour, and connected them into a mixer. In order to obtain a shift from one colour to the other I connected the time-node into the fac-input of the mixer. The time node is set to a frame range from 1 to 100 frames, I thought the time-node would automatically go from frame 1 to frame 100 once it is connected to the mixer, yet in reality the value from the time-node into the mixer did not change at all, the displayed colour stayed the same. 

the main-question: 

What do I have to change in order to make the time-node run from start-frame to end-frame ?
The idea of the Compositor is editing some kind of Input. Either the rendered result via Render Layer Node or an arbitrary image from your hard drive via Image Node. 

At the moment it's only possible to set up an empty plate in the compositor via Mask Node. 


256x256px 'canvas' - automatic animation via Time Node

Note: It's not necessary to use a Time Node (in your case), because it's also possible to animate the Fac value of the Mix Node, related: Is it possible Animate the Compositor node parameters? 


256x256px 'canvas' - Fac value animation