This picture only has black, white and cyan.
How is the [can](https://i.imgur.com/DfRNh0z.png) red? Zoom in to check.
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Vision isn’t what photons hit your retinas, that’s just the start of it. Your brain has to interpret all of these signals coming in through your optic nerves. We see different colors based partially on the wavelengths of lights triggering different photoreceptors in the eyes, but that’s only part of it. If we saw purely based on the wavelengths, things would look like dramatically different colors based on the type of lighting, whether it’s in shadow, how much light is hitting it, etc. While this is a little noticeable (think about something looking a bit different outside vs in fluorescent lighting), colors don’t look drastically different if you put a shadow in front of something.
When these signals reach the brain, they start in the thalamus, then the information is moved to the visual cortex. From there, it is split up in to different areas that process different information, such as processing color, motion, depth and form, although no area does one of these exclusively. This all gets combined to give you an idea of the shape and color that you are seeing. We don’t know exactly how it all comes together, though.
One of the things that can happen during this is that color perception can be affected by borders – basically, you get a bleeding of a color difference as your brain is working out the borders of different areas. This can go one of two ways, either the opposite (contrast), or similar (assimilation). In your example, the broken border is creating a contrast effect. Your brain, while working out all of those pixel edges, is filling in color that is the opposite of the color fill in the rest of the picture. If you concentrate and focus on just the area with the illusory red, it goes away – it’s when you try to process the picture as a whole that it makes your brain fill in those colors.
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