Natural cocoa powder is made from cocoa solids after much of their fat has been pressed out, without the alkalizing treatment used for Dutch-process cocoa. It retains acidity and can react with baking soda in a formula.
Color alone does not reliably identify it. Substitution depends on the recipe's other acids and leaveners; in drinks and uncooked applications, flavor and color may matter more than leavening chemistry.
Reference: Further reading.