Why is dark chocolate dark when pure cocoa is light brown?
There are probably several factors in play:


The chocolate liquor used to make the chocolate may have been dutched (processed with alkali) which makes it darker.  AT the extreme end, it is almost black, like an Oreo (which is made with highly dutched cocoa).
The fat phase surrounds the cocoa particles, and makes them appear darker, much like wetting cocoa powder with water makes it appear darker.
The cocoa used to make the chocolate may be roasted to a greater or lesser degree, which affects its color, might like with coffee.  Thanks to  Didgeridrew for pointing this out.
Milk chocolate is necessarily lighter, as the milk particles are white, and help make the entire bar a lighter shade.


The cacao percentage is not a major factor, as even low cacao chocolates (like the very, very sweet German's Baker's Bar) are quite dark.