It doesn't surprise me that addition/removal trumps colour as the primary cue to change in a visual display. Evolution would favour the detection of "things" rather than discriminating their colour when you think about our ancestors & other animals hunting edible prey and preventing themselves becoming prey. If the animal disappears, you'd have to notice quickly as it is either running away (so you can't eat it) or it's sneaking towards you for it's own purposes. Sure it's important to see whether a berry is poisonous red or delicious purple before you eat it, but that isn't an activity done under time pressure, where the presence/absence of a "thing" is the highest carrier of information. You wouldn't want to trip while stalking your buffalo, but you wouldn't care whether the badly placed object you tripped over was purple or red!