I know someone who has used that armsock tutorial, and it did not work out well, just as a warning. The paint, no matter how well she applied it, was really awkward and stiff and itchy and splotchy instead of giving the nice, smooth grey that she wanted, and they were nigh unwearable because of the texture. We had to make some emergency grey sleeves to go with her outfit, which was awkward, but better than nothing. the technique for making the socks themselves was alright, though I'd recommend not cutting the fingers until after they are sewn, or cutting while sewing, because the pinned and pre-cut fingers tended to slip around and not actually fit her fingers. (She also had a problem with the actual painting process, since the paint gave her a nasty chemical burn, but if you don't have super sensitive skin like she does, then that isn't as big of an issue, though I'd still be worried about someone having that much paint that close to their skin for as long as the painting process takes)
Instead of getting black and painting it, look into getting tights that are already grey. Buy a couple of pairs in case you mess up the first one.