As far as I can tell everything Leigh has said is correct, however I
think that we have "sound reasons" for picturing the sun as yellow.
When I was a kid I was naturally told not to look directly at the sun,
so just as naturally the first thing I did when I had a chance was to
look directly at the sun. I cannot tell you what color the sun was, but
the after image in my eye was a rich yellow. Now I lived in the high
mountains so I had a relatively polution free environment.
Being the dope *ahem* experimentalist that I was, I decided to try other
methods of "seeing" the sun. When a relatively thin cloud passed
between the sun and I, I would again look at the sun. It had a
yellowish orange cast to it.
Obviously in both cases we are not talking about the "true" color of the
sun, but yellowish-orange is visual I have when I think of the sun (from
my observations as a kid).