@fBoyle-- Yes, there are ways to make the henna color come out darker. Prepare your henna paste a few days early. Put it in a plastic container and in the fridge. It becomes darker as it sits in the fridge.
Before applying the henna, squeeze some lemon and add some sugar in it. Dissolve the sugar and keep a few cotton balls aside. After applying the henna, as it becomes dry, apply the sugary lemon juice mixture on the design with the cotton ball. This keeps the henna moist so that it keeps infusing color into skin. Otherwise, the henna will completely dry and start falling off.
Henna should be kept on skin ideally all day or overnight. A plastic bag can be worn over it before sleeping to avoid a mess. After removing the henna in the morning, dab some almond oil or other oil on it. This will also keep the color for longer.