You dip your fabric in the dyepot and when you bring it out it is green. It turns blue as it oxidises. You keep dipping the fabric until it is as dark as you want. The colour fades a bit when you wash it after dyeing and obviously the colour looks darker when the fabric is wet so it is a bit hard to tell how dark the finished product will be.
I thought the first batch of fabric looked quite dark but after washing it and drying the colour was lighter than I'd hoped for. The instructions with the dye kit said to soak the fabric in the dyepot for one to several minutes before removing it so I left it in there for about 5 minutes max.
Now I just need to do something with all this fabric!