What's the best technique to determine if the contents of two images are the same?
A simpler approach (at least to start with) would be to evaluate the absolute difference between the two images and specifically, its mean value or maybe even just the sum of it (across all pixels), and then take its histogram across your sample of pairs of images. 

The histogram will follow a bimodal (or multimodal) distribution with two main modes: The lower mode corresponding to the pairs of images with slight differences and the higher mode (higher values of difference, therefore more error) to pairs of images with drastic differences. You can then use this information obtained from the histogram to work out a threshold. For more information please see: http://en.wikipedia.org/wiki/Multimodal_distribution

A few details:


You don't have to work on the colour images for this, you can simply
convert them to greyscale. The way you do this will depend on the
tools you are using.
The difference image that you depict in your post seems to be a
bitmap, the absolute difference between two grayscale images would
also "look like" a grayscale image, please see:
http://homepages.inf.ed.ac.uk/rbf/HIPR2/pixsub.htm


Hope this helps, happy to amend the response if its not exactly you are looking for, I was a bit surprised to see you going for edge detection straight from the beginning so maybe I have not understood exactly the definition of "frame difference" you might be thinking about.