Why is dark chocolate dark when pure cocoa is light brown? Kind of a simplistic question, but my curiosity cannot be assuaged. When you look at pure cocoa powder, it has a light brown color, like a milk chocolate bar. And yet a dark chocolate bar, which is much higher chocolate content than milk chocolate, is a vastly darker color than cocoa powder. Even a 100% pure cocoa chocolate bar is a very dark color.

Pure cocoa powder



Milk chocolate bar



Dark chocolate bar ~75% cocoa



Dark chocolate bar 100% cocoa



What is at work here? I know chocolate has fats and solids other than cocoa in it, but that just makes it more confusing why a pure chocolate bar is dark as well. Does compacting the cocoa into a solid form (which is how I assume you make a pure cocoa bar) darken it that much? Are there differences in preparing the cacao beans (like different roasts for coffee)?
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.