The caffeine in coffee beans is water soluble..., so it's not the darkness of the beans but the amount of beans, how well ground, and how long they are in the boiling water that makes the caffeine level. The roasting just gives flavor and color. So the old school percolator actually gives you more caffeine from the same amount of ground coffee than if you made the coffee a la espresso. In espresso, the water goes through once, but percolators recirculate the water over and over.
That's why when I made several test batches with green coffee beans that I roasted myself, although the coffee looked and tasted "weak"..., itmademetalkveryfast and gave me the jitters. The "coffee" that I made was full of caffeine, just lacking color and flavor.
LD