but how does each blue see the other blues?
http://www.rsc.org/chemistryworld/Issues/2010/October/SaussuresCyanometer.asp
“how to measure ‘blueness’? Using suspensions of Prussian blue, Saussure dyed paper squares every shade of blue he could distinguish between white and black. These were assembled into a numbered colour circle that could be held up to the zenith at a standard distance from the eye – the matching square established the degree of blue.”
THE ANTHROPOLOGY OF OTHER ANIMALS 