This is an interesting image. The color difference for the horse corresponds to a Delta E of 3, which is just greater than the generally perceived just noticeable difference (JND) value of a Delta E of 2.3. For more on this image see Only Certain People Can See The Hidden Image Inside This Eye Test.
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About Mark Mizen
I have over twenty years professional experience in all aspects of photography and digital imaging. I am Chair of the ISO WG5 TG2 committee responsible for physical properties and durability of imaging material and am currently with HID Global working on systems for security printing for IDs, licenses, and credit cards. Previously, I was Director of Digital Development at Creative Memories from 2009 to 2012 and was responsible for the Creative Memories digital products and services. I also established and directed the Creative Memories Technology Center, which evaluated new products prior to product introduction, assisted with production difficulties, and provided technical information to support product sales.
All About Images Blog 
I had to move my head around a little and thought it might be a cow until I saw the shape of the head. My son, who is color blind, glanced at it and said, “There’s a horse”.