Such properties have no esse except as percipi . I have no substantive quarrels with Rosenthal's arguments, so instead of raising obstacles, I will try to push his claims a little further, and support them with further considerations. But Hume's claim is by no means obvious, however well supported by tradition.
Unfelt pains make a convenient entering wedge.
And if there can be unfelt pains, there can be unconscious sensations of red. Experiments exhibiting similar effects are legion, and while I cannot think of any offhand that have exactly the effect described, I have no doubt that such effects could be produced. Rosenthal questions this claim, but I will go further. We can repeat the Lockean move, and in fact we must. The common wisdom is that modern science has removed the color from the physical world, replacing it with colorless electromagnetic radiation of various wavelengths, bouncing off surfaces that variably reflect and absorb that radiation.