It's got to be cotton.
I dyed mine, starting from a light grayish beige. However, I did preproduction. Just look at the color of the jumpsuit and see what colors are in THAT color. For istance, the prepro suit is gray, green, and blue. Start with the lightest color first. I dyed my suit in that order, first gray, then green, then blue, using RIT powder dye. I believe I used another gray after I did the blue, and that made it perfect. You can use RIT dye remover to fade out the color if you make it too dark. You'll use each dye one at a time, in a separate wash for each. After you are all done with the dyes, run the washer with a load of just like 1/4-1/2 cup of bleach, then with a load of detergent (no clothes in the washer for either load). This will keep the dye from getting on your clothes the next time you do laundry.
If I were to do an ROTJ starting with white, I would use a light tan, then gray, and possibly another color of gray after that (light gray then dark gray). If the color was not exactly right, or seemed to "warm", I would add 1/4 package of blue and see what happens. But...no guarantees. I haven't ever tried to do the ROTJ color.
IMO, the only way to get the correct color is by dying, or possibly by special order fabric if you can ever find it. The fabric color looks streaky, and RIT dyes will give you just that. Dickies do not look right.