You're probably not going to love me very much for this, but I'd say alter your skin tone very little if at all.
I have naturally more olive skin and tan easily, so I have no idea how the different products work. However, I've seen lots of very pale people try to make different things work. In the end, most of the spray/rub on stuff looks fake when applied for more than a tint, and tanning in a bed is just horrible for your body. I'm probably cosplaying Riza Hawkeye next year, and it bothers me that my skin will be too dark, and that, in turn, forces me to use a non-accurate blonde wig. But in the end, the cosplay will look better for embracing my skin tone rather than trying to alter it.
If you still want to try to hit her very dark skin tone, you may have more success in the make-up forums or just testing random products (though this could get expensive). My favorite advice: Slap on some sunscreen and play in the sun! ^_^ The change will be less and take a lot longer, but it will look a lot nicer.