I made mine from white cotton overalls. I used Dylon powdered dye because it doesn't need scalding hot water (hot water from the tap is sufficient). I purchased antique grey and navy blue, intent on mixing them (for ESB's bluish-grey), but the antique grey turned out to have a blue tinge that was nearly perfect on its own.
Being cautious, I dyed it three times, making it a bit darker every time, so I wouldn't overdo it. The end result is perfect. The process was much easier than I expected.
A few tips.
1. Use a tiny amount of dye in a small bucket with some scrap fabric (white cotton) for testing beforehand.
2. The instructions say to leave it for quite a while (I forget exactly; an hour?), but it's basically done after five minutes.
3. If possible, do the fabric pockets at the same time so the colours match.
4. Thread doesn't dye well. If you're modifying the overalls, choose the thread colour that matches your target colour instead of white thread.