I've always used Indian Ink. It shows every little crease and detail nicely and the colour stays true over a very long time. I've tried paint but it doesn't show the lines very clearly. A tip is to buy acid-free paper then it won't go yellow over time. All this stuff is available at art supply shops. Indian ink (artist quality ink) would be fairly safe... it's the stuff they use to fingerprint people... palm readers use it too. Personally I think there are more harmful chemicals in the average bottle of supermarket baby wash compared to using this as a "once off".