Well, I did indeed play around quite a lot to see what's happening when I am doing this. My approach was to reload the image and then go to top view and toggle undo/redo to see the difference between the B&W and the colour version.
To be honest, I didn1t see any difference so I basically trust the method - true that when I go quite close in my area, the imagery is not the best as well as in those rare cases when I use this technique (to paint some small scale, artificial terrain for GE), any subtle difference would be rather unnoticeable.