The read highlighting IS the difference. As per https://support.smartbear.com/testcomplete/docs/working-with/managing-projects/properties/visualizer.html, if you have a visualizer frame attached to a step during recording and then you have a visualizer frame attached during playback (visualizer capture turned on for both), a comparison is done. There is a button in the "Actual Image" panel that, if you have it selected to "View Comparison Result", will show the red as you have shown in your screenshot.
Deselect that and you should be able to see the actual image without the red pixel overlay. See https://support.smartbear.com/testcomplete/docs/testing-with/visualizer/comparing-images.html for more information
