Here are 2 articles by Stockholm IPMS memebrs
Article by Fredrick Lundbeck Article by Anders Isaksson There was a thread on here, not too long ago when this was discussed. The salt technique was discussed and is another possible method.
I personally have tried the first method by Fredrick Lundbeck t show a desert panzer II wearing throught the sand colour to the dark grey underneath. Worked really well, but I think you should paint some sprue first and try this out on the sprue. Gives you a better idea of what to expect, gives you a better idea of how well the paint comes off (try different areas after every 10 minutes to get the best result for your climate) and build your confidance for doing the model.
Hope it turns out well. Keep us informed!