do the bees digest the old wax and then it comes back from the gland to be used again? I know wasps just salivate the wood and chew it.
No, but they will re-use it. Such as the wax from foundation (whether wax foundation or sprayed on plastic) they will draw that out into the comb. However, they don't seem to collect it from outside sources, only inside the hive/ on the comb.
I had a frame that I wanted to use for cut comb that had dark comb in it that I cut out, leaveing a little bit at the top for a guide. They actually drew the frame out nice starting from the top, but there were dark areas drawn down from the little bit of dark comb that I had left at the top. I didn't want that in my cut comb :(.
I think people have show this too using colored wax for foundation...the whole comb will have that color in it, not just the foundation.