I haven't actually opened the picture in Photoshop, but I'll take at face value the claim it has no red pixels.
But, what does that actually mean?
Photoshop's default on color pictures is to present a photo in "RGB" format. That's Red, Green, and Blue — the three primary colors. But, many Photoshop commands let one manipulate not only those three colors, but the three commonly accepted secondary colors of Cyan, Magenta and Yellow, plus a channel for Black. If you've ever heard of CMYK (B already used for Blue) photo editing, that's where it comes from. The Black channel is necessitated by the conversion of primary colors back to secondary.
Anyway, back to RGB colors. Several thoughts here.
First, many colors, even in "normal lighting," whatever that is, aren't what we think. If you use Photoshop much, you'll see that the "green" in grass is about one-third yellow.
Second, does Photoshop's 256-level format (two to the eighth power of bits on eight-bit basis) for each color channel imply a level of digital accuracy that doesn't exist? I mean, we can peg light to 500 nanometers. But, is that blue or is that cyan, or turquoise, if we use a non-technical color, or what? Wiki has a full piece on "spectral color" which raises such issues.
And, with that, we're into various issues of philosophy.
Setting aside some aspects of epistemology, we've got what would be either informal logic or linguistic philosophy, first. That is the issue of categories and definitions.
Per what I posted above, where does one color "stop" and another "begin"? That's not a science issue, that's a philosophy issue.
We also have linguistic philosophy issues on how one defines color. Setting aside the Sapir-Whorf hypothesis, which is nonetheless of a certain degree of strength, some languages distinguish between more colors than others. Isaac Newton, with the rainbows produced by his prism, famously distinguished indigo as a seventh color of the spectrum.
To the degree the above photo is an optical illusion, it trades on something else which is related to epistemology, and to David Hume's project of empiricism. That is the idea of qualia, or why do things seem to be the way they are.
The idea of qualia, if accepted in one of its several forms and definings, undercuts the "blank slate" idea of human perception stressed by Hume, and to fair degree by fellow empiricists. A child old enough to point to a red splotch in normal light, when shown this picture above, would not be able to point to claim it has a color similar to that red splotch on the blank slate theory of the human mind. (I frame the example this way to try to bracket the issue of the baby's mind being "contaminated" by explicit written or oral conversation.)
As to opponents of the idea of qualia? As I've gotten older, and more read in modern philosophy, I find Dan Dennett's arguments more and more lacking. The more and more we do current research in robots, artificial intelligence and similar, the more and more we realize human minds don't work that way. Dennett's other objections are somewhat functionalist in nature.
A side issue is that discussions of qualia often get wrapped up with issues of ontological dualism, even though in reality the two are separate items.
Or IS it ultimately a philosophical, not a scientific, question?
|Nature painting by tetrachromat Concetta Antico|
Per what I said above about a lot of green in Photoshop actually being yellow, I can see, looking at her paintings, how tetrachromat Antico, at the link above (learn more about her at her website), has her fourth cone cells tuned to somewhere between red and green (which actually shades to a yellowish-green in most people anyway).
We mustn't steer away from the ditch of scientism only to drive into the other ditch of philosophism.
Nor can we consider many scientific questions outside of philosophy.
On the other hand, we also can't consider many philosophy questions outside of science. Philosophers fear, often rightly, "poaching" on their territory by the science world. On the other hand, in many issues related to consciousness, volition and other matters of the human mind, philosophers all too often remain resistant to legitimate claims of neuroscience and related fields.