Should this be considered color vision? It seems safe to say that whatever the qualia of scanning chromatic-aberration vision would be, it would be very different from our simultaneous realtime trichromatic color vision. And it’s worth noting that under the Stubbs model, to explain why behavioral assays (not just the finding that they only have one kind of photoreceptor) found them to be color blind, there’s a lot of things they can’t do with the chromatic aberration trick:
Second, some behavioral experiments (7⇓⇓⇓–11) designed to test for color vision in cephalopods produced negative results by using standard tests of color vision to evaluate the animal’s ability to distinguish between two or more adjacent colors of equal brightness. This adjacent color comparison is an inappropriate test for our model (Fig. 4R). Tests using rapidly vibrating (8, 9) color cues are also inappropriate. Although these dynamical experiments are effective tests for conventional color vision, they would fail to detect spectral discrimination under our model, because it is difficult to measure differential contrast on vibrating objects. These results corroborate the morphological and genetic evidence: any ability in these organisms for spectral discrimination is not enabled by spectrally diverse photoreceptor types
...In our proposed mechanism, cephalopods cannot gain spectral information from a flat-field background or an edge between two abutting colors of comparable intensity (Fig. 3). This phenomenology would explain why optomotor assays and camouflage experiments using abutting colored substrates (7, 9, 11) fail to elicit a response different from a flat-field background. Similarly, experiments (10) with monochromatic light projected onto a large uniform reflector or training experiments (8, 9) with rapidly vibrating colored cues would defeat a determination of chromatic defocus
...We predict that the animals will fail to match flat-field backgrounds with no spatial structure as previously shown in figure 3B in the work by Mäthger et al. (7) just as a photographer could not determine best focus when imaging a screen with no fine-scale spatial structure. If, for instance, their ability to spectrally match backgrounds was conferred by the skin or another potential unknown mechanism, they would successfully match on flat-field backgrounds. However, under our model, they should succeed when there is a spatial structure allowing for the calculation of chromatically induced defocus, such as in our test patterns (Fig. 4) or the more naturally textured backgrounds by Kühn (21). If, however, cephalopods truly cannot accurately match their background color but solely use luminance and achromatic contrast to determine camouflage, we would expect the response on colored substrates to be identical to that on a gray substrate of similar apparent brightness with identical spatial structure.
That’s a good question, and the answer may be that they don’t have color vision in any normal sense; what they have is the ability to use chromatic aberration to focus their eyes for various colors, and this serial focusing scan lets them decide how to adjust their skin to match surroundings: “Spectral discrimination in color blind animals via chromatic aberration and pupil shape”, Stubbs & Stubbs 2016.
Should this be considered color vision? It seems safe to say that whatever the qualia of scanning chromatic-aberration vision would be, it would be very different from our simultaneous realtime trichromatic color vision. And it’s worth noting that under the Stubbs model, to explain why behavioral assays (not just the finding that they only have one kind of photoreceptor) found them to be color blind, there’s a lot of things they can’t do with the chromatic aberration trick: