If the difference in two images is only the alpha channel but the rgb channels are the same, the difference image won't show that anything has changed.

Example

In this case it's a UILabel that has where the textColor is black (rgba(0, 0, 0, 1)) and the backgroundColor is clear (rgba(0, 0, 0, 0)).

Reference

Failure

Difference

If I take the reference and failure images and pass them to ImageMagick's compare command, it gives the following image showing the differences..

If I change view being snapshot to have a white background, then the created difference image shows the changes. While this does work, adding a background color to the view then means it isn't exactly the same as the view that'll be used in the app.