I have just discovered this very nice method to insert keyframe to a light when we are in the Cycles Render mode so that the lamp will change color from one frame to the next. Here is how to insert a keyframe to the light.
i) In cycles render select the light by left clicking on it.
ii) Before you do anything make sure you are at frame 1 by looking at the timeline editor.
iii) Select the object data tab of the lamp.
Under Nodes click on the Use Nodes button then change the color of the lamp to any color, next press ‘i’ when the cursor is on the color box to insert a keyframe. After you have inserted a keyframe to the lamp the color box will get a border which indicates that you have successfully inserted a keyframe.
iv) You can now move to frame number two under the timeline editor and insert the next keyframe using the same method.
v) It is simple to change the color of a mesh by just changing the color of the lamp as mentioned above!
Now switch to the Rendered viewport and press Alt-A to render the animation and see how the lamp changes color through time.