Still more on the color illusion

Well, a reader showed in an email that that the hearts in the illusion below really were very slightly different colors due to the compression algorithms used in making the image. (However, those slight differences do not account for the striking perceptual effect).  The reader who demonstrated this anomaly won an autographed book from Matthew.

HOWEVER, reader Mel made his own illusion, so we know that here the colors of the squares really are identical. His notes:

To demonstrate the phenomenon a bit more cleanly I constructed the following image using a spreadsheet. I used very small cells (0.3 cm x 0.3 cm) and filled them with various colors and then took a screenshot. The image still shows the illusion and only three colors were used in constructing the image (magenta, orange, blue-green).

And another demonstration using magic markers. I think the efficacy of the lines in fooling viewers about the color has been shown. We’ll now leave this illusion behind and move on.


  1. BobTerrace
    Posted February 18, 2018 at 8:03 am | Permalink

    But the real question is what do color blind people see? Different shades of gray?

    • Peter
      Posted February 18, 2018 at 8:14 am | Permalink

      There are a number of websites that allow uploading of files to simulate the various forms of color blindness – or color deficit:

      Try uploading the hearts image to see the results. People with color blindness do not see in black and white.

      • nicky
        Posted February 18, 2018 at 2:21 pm | Permalink

        There are some rare forms of colourblindness that only see in scales of ‘grey’ (complete lack of functional cones), however their visual acuity is so bad that the lack of colour vision is but the least of their visual problems.

      • Posted February 19, 2018 at 6:01 am | Permalink

        I have deuteranopia, probably the most common form of colour deficiency (generally referred to as “colour blindness”), and can see the illusion very clearly. Monochromatic vision (greyscale black & white) is extremely rare. check out the Vision simulator to get an idea of how colour blindness affects sufferers.

    • gravelinspector-Aidan
      Posted February 18, 2018 at 1:24 pm | Permalink

      Up to 200 odd shades now, isn’t it? Unless the books, plays, movies and (probably) board games are getting a bit repetitive.

  2. Posted February 18, 2018 at 8:17 am | Permalink

    What happens if the sqares are placed touching each other just offset by the vertical striping?

    • Posted February 18, 2018 at 9:48 am | Permalink

      You don’t need to offset by any vertical stripes. The “blue” square interrupts the orange lines, which is what makes it look blue, while the “green” one interrupts the magenta lines. So they could be directly adjacent if you want. I’m pretty sure it would look “the same” minus the separating distance.

      • Posted February 18, 2018 at 11:42 am | Permalink

        Are you able to try that kind of stuff on your computer? Would love to see.

    • Michael Fisher
      Posted February 18, 2018 at 1:24 pm | Permalink

      Cicely, here is the same image with the boxes adjacent:

      • gravelinspector-Aidan
        Posted February 18, 2018 at 1:33 pm | Permalink

        Actually, for me, that one doesn’t work. Same green, both sides of the shift.
        The spreadsheet version up-thread gives me a … I’m not sure of the name of the effect. When you look at the traditional drawing of a wire-frame cube-corner and your perception flips between being a view of the interior and of the exterior of the box. Well I get that same sort of flickering between the left box being green and the right one blue, and vice versa

        • Michael Fisher
          Posted February 18, 2018 at 1:37 pm | Permalink

          Necker Cube? For me the greens remain different. And no flicker or disorientation with the original.

          • gravelinspector-Aidan
            Posted February 18, 2018 at 1:48 pm | Permalink

            Necker cube – that’s the one.
            Lots of individual variation in optical systems. I remember being blown away by the prediction and then discovery of human tetrachromats.

          • Posted February 18, 2018 at 4:46 pm | Permalink

            For me the two greens are very slightly different. I can kind of feel a tension in perception wanting to make the two sides into a uniform background?????
            So a differences in perceptual systems.

            • Michael Fisher
              Posted February 18, 2018 at 5:09 pm | Permalink

              It’s Gravel drug usage rewiring 🙂

        • eric
          Posted February 19, 2018 at 7:26 am | Permalink

          Me too, I see it as the same green. Which now makes me want to look at images with a 2-stripe gap, 4-stripe gap, etc… to see at what point the illusion kicks in.

          • Michael Fisher
            Posted February 19, 2018 at 8:02 am | Permalink

            The gap matters a little bit
            The strongest factor is the width of the stripes – narrower the better, See here:


            • Posted February 19, 2018 at 8:21 am | Permalink

              The black background also enhances the illusion.

              • Michael Fisher
                Posted February 19, 2018 at 8:25 am | Permalink

                Here’s the same without black background:

              • Posted February 19, 2018 at 12:10 pm | Permalink

                Hmm, the top right image-looked at from a distance clearly is blue vs green but from close up it is easy to read it as a uniform green background. Oh well. Maybe I should read up on variability in human retinas of numbers of cells sensitive to different wavelengths.
                Thanks for all that computering.

      • Posted February 18, 2018 at 4:27 pm | Permalink

        Hey, thanks,
        Neat. What would that last blue/green stripe appear as-in between the two contiguous orange and purplish?

        • Michael Fisher
          Posted February 18, 2018 at 5:12 pm | Permalink

          OK. I’ve done my best [considering wine consumption] with these two rearrangements:



          • Posted February 19, 2018 at 6:05 am | Permalink

            Very interesting, thanks!

          • eric
            Posted February 19, 2018 at 7:28 am | Permalink

            Thanks for posting those. I see slight difference in color at the “ends” of the squares in the first image, but practically none in the top/bottom image.

  3. Posted February 18, 2018 at 9:44 am | Permalink

    I think it’s awesome how your brain automatically compensates for differences in ambient lighting and partially obscuring obstacles, to show you how each object contrasts with its environment. The fail here is much less impressive than the routine success. If only I could make my machine vision algorithms at my job work so well.

  4. Derek Freyberg
    Posted February 18, 2018 at 11:05 am | Permalink

    On a slightly different but related (these illusions reside in the brain even though they are visual), NPR yesterday had a show on neuropsychology. One aspect was face recognition, and morphed faces (two faces morphed together by software). Apparently, if you morph yourself and someone well-known, you see yourself in the morph, others see the well-known person. Even stranger, if you anesthetize different sides of the brain and look at one of these morphed photos, if your left brain is anesthetized, you see yourself; but if your right brain is anesthetized, you (like everyone else) see the well-known person. Weird things go on in the brain.

    • Posted February 18, 2018 at 11:54 am | Permalink

      Yes, indeed. Possibly the same kind of experience as when you are expecting a friend to showup and you see them as you expect them to be- left brain? But sometimes catching a glimpse unexpectedlythe same friend can look completely different-especially if you have known them for a long time and they have aged or otherwise changed. The left brain goes with the built up expectation and the right brain presents a more up to date image. It can be quite startling. ????????? Or if you catch an unexpected image of yourself as a reflection in a store window..

      • Michael Fisher
        Posted February 18, 2018 at 1:08 pm | Permalink

        I don’t trust that left brain / right brain stuff – there is a lot of hype on the subject. It’s particularly difficult to untangle with respect to vision where some signals cross to the opposite brain hemisphere & some do not. And vision processing is a multi-step operation some of which is done before it hits the brain proper.

        • Posted February 18, 2018 at 4:20 pm | Permalink

          There is some validity to the right brain left brain stuff-and a long way to go to understand it better. So I take your point.

  5. gravelinspector-Aidan
    Posted February 18, 2018 at 1:27 pm | Permalink

    We’ll now leave this illusion behind and move on.

    Optimist! See up-thread.

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