i picked up a couple of cheap Luth AR stocks, but they're pink
i dyed some parts a couple of years ago and didn't have great luck.

can i get to a real black with dit dye? do i use black dye or another color dye? i read some people have used chocolate brown and graphite...
is there a special polymer dye or just regular rit dye?

thanks

Rit has a line of dyes for synthetic fabrics. I would try that if I didn't want to spray paint.  

ETA: https://www.ritdye.com/type/dyemore-for-synthetics/

Black Rit Dye for synthetics will work (as long as the polymer itself is pink, and it isn't a pink coating like cerakote).

I've dyed FDE polymer with it before to turn it black, it turns out pretty close to true black, but appears very dark navy blue under bright light.

Keep in mind that the dye only penetrates about 1mm or so, so if it gets scratched you might need to redye, or touch it up with a sharpie.