Green coffee beans contain more sugars, acids and caffeine than dark coffee beans. For example, they are rich in Chlorogenic Acid which has strong antioxidant properties which is beneficial to the hair! Dark coffee beans don't have quite the same benefits as green coffee beans, however that isn't to say that this ingredient can't be used, it just might change the efficacy of the shampoo. As this recipe hasn't been formulated in this way we cannot predict what the result would be. Therefore, if you do choose to try it, perhaps only make a small amount so as not to waste the ingredients and always do a patch test first!