There's cocoa and cocoa and cocoa.
Each "cocoa powder" available has a different fat content, colour range and taste. Has to be 100% Cocoa, there will be zero sugars/fermentables but the colour and fat (flavour) will still vary. It mainly depends on the roasting the bean has been through prior to milling, much like coffee. If it is not 100% cocoa there will be milk solids and various sugars in it. 100% cocoa is amazingly bitter, you wont be able to eat it.
As suggested, try it out to see how the colour goes. Try to get a powder that you can regularly get so you can be consistent with batches.
Edit: Sory, just had a look at one of our Spec sheets (I work for a company that sodl cocoa) and there are potential fermentables even in cocoa.
Per 100g of one particular grade, not gospel for all grades
Total Carb 34.2
Sugar 2.0
Cellulose 17.5
Starch 11.70
Pentosanes 3.0