Dimensionality
Who's the brightest?
Shades of blue (or shades of grey) constitute a one-dimensional system.  The puzzle is solved by putting them in a line.
The color seen through two-cone colorblindness requires a two-dimensional array.  Your computer reproduces these colors with only the green and red phosphors.  The blue phosphor is not used.
Normal human color comes from a three-cone system.  It requires three dimensions to solve the puzzle.
Color as seen by a bird requires four, five, or six dimensions to solve the puzzle.

To a bird brain, all humans are profoundly colorblind.  A human being cannot know six-factor color as a bird knows it. 
 
 

It's beyond the edge of human comprehension.

How was such a thing discovered?  How can we ask and answer questions about the color seen by a bird?  What is the practical significance of this fact?  What's the practical significance of the common human colorblindnesses?

Explore further into Colorblind Territory