I use a combo of Freddy333's Iodine method. However, Iodine on Lumi does some strange stuff. It is as if the Lumi "eats" the Iodine, so it goes on dark and returns close to normal. Bizzaro. And you will never totally kill the lumi, it will shine through a bit still.
I will take a Lumi hand, and ON THE BACK of it, color it with a "Brown Sharpie" marker. The brown will darken and show thru to the front of the hand.
The below hands were Sharpied on the Back, and Iodined on the front with a manicurists Q-Tip.
The DOT on the 24hr hand was Sharpie added to the front side, and smeared with Iodine.
You can also Try Coffee or Tea Staining. But beware, too wet and you might disolve the Lume all together.