Echo what lady bedlam said. I Tried to dye 65/35 poly cotton blend. You will NOT be able to do it with RIT dye. I tried. I even used that exact Ghost buster tutorial which is horse crap.
One option ( i have not tried) is to use a synthetic dye. Dharma trading is the way to go for buying it.
If your wanting gray coverall just buy gray coveralls. SPeaking from expierince as I went thorugh 3 flight suits trying to dye them. Just buy the coveralls they have light and dark gray. Nother linky below.
Good luck.
Thanks for letting me know about that tutorial! Saved my life, lol. But I would buy a new one, and I might, but I don't think they'll have the Dark Grey in my size (Medium short I think). They have Dark Olive or something like that but I rather not have the green tint to it. Plus it's expensive considering I got mine for $30 and I still have to buy a vest for $80 and my helmet for $150. It might not sound like a lot but I'm a high school student with a minimum wage job working two days a week

In the summer is when I make most of my money as I work for my family during then and hopefully, especially in July, I'll be able to drop a couple hundred easily on my kit so I might buy a gray one then. Especially since I won't be able to apply until August (18th bday). But thank you for all the advice you've given about the tutorials. I really appreciate it!