You'd have to track the movement of the light (Postition, Rotation, and Scale) - I'd use Mocha. Put a masked layer over the light and apply the tracking data. Mess with the layer using Hue and Saturation and layer styles. See this video for tracking:
Here's a series of clarifying questions that may lead to an alternative to AE:
What's the form of this stop light image: a still? Is it original art, perhaps? What kind?
Could it be video that's been shot already?
If it is indeed something shot, is it a stoplight hanging over an intersection on a wire, or one on a pole on a street corner?
If you're using video, I'd go for a reshoot if necessary to get a couple of cycles of green-yellow-red on a locked-down camera. Then using simple editing in an NLE, you could get the lights to change whenever you wanted, and it would look absolutely real, because it IS absolutely real.
Sr. Promotion Producer
KCRG-TV (ABC) Cedar Rapids, IA