It's more difficult, but it can still be done. I dyed these two Eleora wigs- not a Cher, but still the same fiber- from silver-white with sharpies, and they're both Eleora fiber.
(Crappy photo. The flash was making the netting colour show through a bit)
(Both wigs were pretty damn close to what I wanted, but it took several more prismacolours markers than I'd planned- the formula for Zexion finally ended up at 3 blue slate, 2 periwinkle and 3 40% cool grey, and prismacolours have a bit more ink than Sharpies, too. Magus wasn't too different, just more blue than grey)
I don't recommend Eleora if you need a really vivid colour, that's for sure. They also run easier when you wash them, so they have a shorter life than other wigs (after setting eleora fibre with the dye and such and styling, the fiber kind of loses it's softness even after washing, and because dye runs out easier you'll need to give it a light re-dyeing each time it's washed. Once that's been done two or three times, the wig's just crunchy and gross.)