The two computer screens depicting completely different subjects have almost everything in common, in that they are of the same material. However, where they differ -- namely, the color of each pixel -- is where all the information about the painting is contained. So the screens have enough different information (at the quark level) to distinguish what the paintings are about.

So I don't think you are getting at why "about-ness" isn't related to the quarks of the painting. I think a better example is a stick figure. A child's stick figure can be anybody. What the painting is about is in her head, or your head, or in the head of anyone thinking about what the painting is about.

So it's not in the quarks of the painting at all. "About-ness" is in the quarks of the thoughts of the person looking at the painting, right? (And according to reductionism, completely determined by the quarks in the painting, the quarks of the observer, and the quarks of their mutual environment.)

Above, you wrote:

there's also nothing legitimate on the level of quarks [of the painting] that could be used to differentiate between a painting that has a subject and a painting that is just random blobs

Thus I agree with this statement as it is written, because I think the difference in the subjects of the paintings are found instead in the thoughts of the beholder. Would you agree that there is a legitimate difference at the level of quarks between the thought that a painting has a subject and the thought that a painting is just random blobs?