I'd be careful with using Nightshades, the stuff works good, but DO NOT use more than one light coat or you'll make your lights effectively worthless. I used it to tint the lenses of the gauges on my a-pillar (I forget specifics, but wiring in resistors wasn't dimming it in the traditional sense), and went with a couple quick coats, they're hard to see during the day. If you do go that route, practice on something else first.
My idea, you might try having some paint mixed up. Like 10 parts of clear to ONE part of a darkish red. They're a cherry color on JDM cars, not really tinted per se... Just remember that looking cool with darker lights and getting rear ended because someone doesn't know you're hitting your brakes are two closely related things in this case.
