I use regular fabric dye though I'm currently looking for something a bit stronger as the pouches still tend to fade lighter over time and something about rattle canning canvas just doesn't seem right, IMO. Also something worth noting, most canvas MP-40 pouch MFGs use nylon stitching which doesn't dye. I had to use a sharpie for that.
I would be interested in knowing if you find something stronger, something industrial that doesn't cost a arm or leg like convertible top dyes YIKES!