So, the first chair cover is a beautiful rich brown color. Looks better than it looked on the package.
My dye method the first time was as follows:
1) put cover and one packet of dye into machine, set it on heavy cycle (57 minutes) with hot water
2) five minutes later, realize I forgot the cup of salt, added cup of salt and some warm water into detergent dispenser
3) hit "pause" when there was 43 min left on timer, leaving stuff to soak in wash water
4) waited five minutes
5) realized one packet of dye wasn't enough for the weight of fabric in there
6) dissolved another packet in water, poured it in through the dispenser along with a lot of water since there was dye everywhere, spilled a little bit on the floor
7) noted that the dye-adding process took five minutes
8) waited 10 minutes
9) hit "resume" so the washer rinsed and spun the stuff
10) took it out and it was perfect
The second time, I put two packets of dye and the salt into the machine, ran it through the heavy cycle. I took it out and it was way light, and was a weird greeny olivey color.
Which was weird, because the chestnut color I chose was on the redder side, and I chose it rather than the brown because the brown was too green. I thought if anything it was going to be too red, which was preferable with my other living room furniture.
So I ran it through again with a third packet of dye and another cup of salt. I figured if one was darker or whatever, I'd just wash all of it together and let it bleed to a compromise. Only it came out and now is a very dark awful olivey green. WTF? Now I'm washing it with detergent to see if I can lighten it to at least a decent color, and maybe just have non-matching (but both in decent colors) chairs. If not, I'll craigslist this cover or something and buy a new one (hey, they're only $30!) and try botching the dye job as badly as I did with the first one, since that came out so beautifully. If it still is messed up, then I'm going to e-mail the iDye people and ask them WTF was wrong with my dye.
EDIT: Now that it's dried, both colors are fine, but they're different:

I almost think I like the right one (the one that was previously too green) better. I put all of it in the washer together with hot water and lots of detergent to see if I can get them to bleed and even out into an in-between color. We'll see what happens. Otherwise, I'm going to get a new cover and dye it and use whichever two are closest. Then I'll dye the couch covers. I don't care if they're a bit different from the chairs, but I do want the identical chairs to be basically the same color.
YET ANOTHER EDIT: They didn't change color whatsoever when I washed them together. I'm out of dye (as is the place where I bought all of theirs) but have ordered more.
Options I'm considering:
1) Dye the redder one with one more packet of dye, since it has less dye in the fabric overall and seems to be the one that got into a batch of dye that's a bit off.
2) Dye all of it together with another few packets of dye, hoping I can get them both to their saturation point.
3) Dye one more cover and use it along with whichever original one is closest.
4) Dye two more covers in the washer together and give away these two to different people.
5) Put the covers on the chairs and see how it looks.