Blond hair has very low levels of the pigment eumelanin.
Is that why it appears to light?
It is, Eumelanin is a dark pigment.
So I suppose that means that brunette hair has lots of Eumelanin. 
Correct! the lack of Eumelanin results in a yellowish color.
What does that mean for people with red or auburn colored hair? 
Some red hair involves Eumelanin but it also involves a different more reddish hair pigment. Some blond people are referred to as strawberry blond.
That is very interesting! So Strawberry Blond people must have small amounts of Eumelanin but also the other pigment.
Correct! in fact only 1 to 2 percent of the world population have red hair.
It is very rare, that is for sure. Does grey hair have any Eumelanin at all?