Hi Eily, I had the same problem and actually gave up colouring silver for a while but I have found a couple of answers. One is eggs. My jewellery is vegan so I don't use this method but I have tried it (with eggs from my exbattery hens) and it does work, normally. The best way I found is to hard boil some eggs, just try two or three to start with. Then peal them and mash them up in a microwave proof container. If they are still warm put the silver in the mash. The mash needs to be warm, if it cools remove the silver and put it in the microwave and warm. I found this method to be messy and wasteful. It doesn't give the best result on silver either, but you do get some interesting browns and patterns.
The best thing I have found is sulphur solutions sold at hardware stores for controlling fungal diseases on plants. Most of these are ok to be used in organic gardening as the ingredients are also used by plants as a natural food. They last a lot longer than L of S if kept in a sealed container out of the light and they are really easy to use. To wash the silver off use an outside tap and a bucket. That way you can spread out the diluted sulphur on your garden rather than sending it down the sink.
If you need any more info just ask.
Utopian Creations eco jewellery and supplies.
Ben, thanks soooo much for helping me with advice so I can spread my greener existence into the work shop!!!! I still have a long way to go but I know every little bit helps. I am realy ispired by the way you work and your enthusuasm towards doing what you believe in. Good on you!!!!!! Cheers, Lauren