I emptied the last of the édes paprika into my hair today, and topped it off with some csípős. I can't say whether the Hungarian-ness of the paprika actually makes the henna redder or not, but the peppers of a nation famous for drinking bull's blood really ought to be the reddest. I suppose Spanish paprika might also contain a lot of bull's blood. Édes (EH-desh) paprika is made from sweet peppers, whereas csípős (CHEAP-ush) is spicy, more like cayenne.
As a general matter of principle, one ought to add as many reddening potions to one's hair dye as possible. I've concocted henna recipes with all manners of paprika, cinnamon, cloves, cayenne, hibiscus, coffee, red wine, and human blood -- all with flaming success. I mix my henna with strong black tea (and reddening potions) to the consistency of algae. Then I let it sit somewhere warm, covered with a plastic bag, to ferment. The next day, I pull a garbage bag (with a hole in it) over my head, add some olive oil to the henna, lather myself with coconut lotion wherever I don't want henna to stick, and start globbing it on to the dirty blond roots of my hair with a comb. I pull the garbage bag up over my hair, secure it, and wrap it all up in a towel to incubate for several hours, during which I may or may not watch too much Cowboy Bebop.
I rinse it out into a bucket, and empty that down the toilet (or water the garden; it clogs bathtub drains just like you would expect a gritty green slime to do). Applied properly, henna should leave your hair smelling of new-mown hay and blazing with the color of a thousand flaming, bloody peppers.