If you suffer from a more common form of colorblindness, you won't be able to make out the symbols in the above picture. In an example like this, I've always assumed that a colorblind person must just see brownish red everywhere.
Then again, maybe he or she just sees green. Maybe gray. Then I got to wonderin'...
Color wavelengths are real and measurable. When I see a red barn, you see a red barn, and we can agree on a color because we're sensing the same wavelength. But what if I switched brains with yours and discovered that your perception of red is more like my perception of green! Because we can't swap each others sensing organs, we'll never know if our consensus about a stimulus is exacting or if we're thinking analogously.
I wonder what kind of mental relativity is at play. How different are we, really, from one another, and how can we ever know?