Your tongue gets coated as toxins leave your body! The cleaner your body gets, the pinker your tongue becomes. If you were to eat, your tongue would become pink almost immediately...or at least that was my experience. The book says that ten days is enough to do your body good, but some say that it is also a good idea (if you can handle it) to go longer...until your tongue becomes pink and the toxins are gone! I went 10 days in April, and my tongue remained coated up until the last day. I plan to go for 21 days this time! You asked why 21 is a magic number...I think because it is exactly 3 weeks. There really isnt a "magic" number, just listen to your body and do what is best for you!
I too am relying on God to sustain me. What I REALLY wanted was for Him to just relieve me of my food addictions to Sugar and carbs! ...but its me that put that bad stuff into my body, so it will be me who takes it out. I also believe that I am learning how to be more disciplined and to have more self control. God often wants us to "go through" the process in order to reap the benefits. Good luck!