Of course, your brain could just alter what's written on your arm.
When you observe the world, your eyes aren't just lenses that coldly record whatever is in front of you. Instead, your brain adjusts the image for context from moment to moment. And in this case, it's filtering what you see based on a built-in knowledge of what shadows are.
Your brain understands that the floor is a checkerboard pattern of dark and light squares. It sees that there is an object casting a shadow over it. It knows that square B is still one of the "light" squares, even though it's the same color in the shadow as the dark squares are in the light, so in order to avoid confusion, it automatically adjusts the lightness of square B for you before that shit reaches your consciousness. It's like you've got an entire Hollywood VFX crew living in your skull, fixing the world in post-production before you actually get to see it. Keep that in mind the next time you hear some eyewitness testimony.
We could give you endless examples of this trickery. Take these two yellow lines, which are exactly the same length:
Don't believe us? Go try it on an actual railroad track!
Most people see the top line as being bigger, obviously because it thinks that one is farther away due to the "tracks." And even beyond the visual, you're unconsciously adjusting the world for context all the time. For example, if you hold a large and small box of equal weight, you will perceive the large box as lighter. Or, if you drink soda after eating cake, it will taste less sweet.