I never use any extract but light. Let the specialty grains do the flavoring and coloring. When you make a stout, the dark grains is what gives it the flavor, and it will be as dark as you want it. As an example, my stout is 1/2 lb roasted barley, 1/2 lb. chocolate and 6 lbs light DME. Black as can be.
If you have a recipe that calls for dark or amber DME, just use your light an pretend it is amber or dark . Follow the recipe the same way, the specialty grains will make it happen.