It isn't food coloring, it's Dutch process - mixing cocoa powder with alkali makes the cocoa powder darker, and repeating the process makes it darker and darker

It isn't food coloring, it's Dutch process - mixing cocoa powder with alkali makes the cocoa powder darker, and repeating the process makes it darker and darker until it's black. It alters the taste too, making it less bitter - Oreo cookies are an example of Dutched chocolate. It's possible to have an 80% light-brown chocolate bar, but the cocoa is more bitter than the darker dutched cocoa, so you don't see lighter-colored chocolate bars at 80%. Milk chocolate is typically not made with dutched cocoa powder, and it appears lighter in color, and has more sugar and milk to compensate for the more bitter flavor of the non-dutched cocoa powder.