Re: When two people see a colour, how do we know they perceive the same colour?

Wow!  I learned a lot researching this question!  I thank you for my 
self-taught short course in philosophy.  This question is more in the 
philosophical than the biological realm, and I'm sorry to say, philosophy 
has no clear answers for you.

On the plus side, you have posed a question that philosophers have been 
discussing and debating for literally thousands of years.  The Sophists of 
ancient Greece were the first to tackle the question of perception question 
directly.  One Sophist named Protageras of Abdera, proposed that humans can 
know their individual perceptions of things, but they can never know the 
things themselves.  By extension, this also suggests that they can never 
know the perceptions of others.

In modern philosophy there's a whole field called "theories of color" and 
another called "theories of color perception". There are many schools of 
thought on the nature of color, but most of them break down into the 
difference between objective and subjective perception.  Some philosophers 
believe that what is seen are the physical properties of the object itself, 
including its color.  This is the objective view of perception.  The 
subjective view is that the apparent properties of an object (or what it 
looks like to us) are dependent on how our mind interprets the object.

Whether you think you see what everyone else sees depends on which of these 
two philosophies you like more.  The objective view suggests that everyone 
sees the world the same, and the subjective view suggests that everyone 
sees the world slightly different.  As a neuroscientist, I prefer the 
subjective view.  I prefer this view mainly because everything we see is 
processed by our brains, and everyone's brain is slightly different.  In 
general, all people probably do see the world similarly.  If I give you 
directions to build a red box with green stripes and then I come look at 
it, I will see a red box with green stripes, even though it was you who 
selected the colors.  Of course, this doesn't eliminate the possibility 
that if I had your eyes and brain, the color you selected as red I'd call 
green.  But nothing really eliminates that possibility.  After all, have 
you ever wondered what a colorblind person sees - how could red and green 
look like the same color?  Hence the field of philosophy.  

Please keep in mind that I'm a neuroscientist, and not a philosopher, so my 
summary of 3000 years of philosophical reasoning is not complete.  For more 
discussion (by real philosophers) of this and other issues, there are many 
websites.  The one I found most useful in researching this question was:  
http://www.artsci.wustl.edu/~philos/MindDict/index.html,
 which is a 
Philosophy of Mind website (try searches under "perception" or "color" for 
some good explaanations).  Good luck, and please contact me if you have any 
other questions!

Brenda