I have a large ceramic room. We had 3 large recycling bins that we inherited from many years of folks ordering different types of clay. Not a good recycling scenario. Lots of problems with this system with breakage etc. From what I have read it is best to keep what you are recycling to be the same clay body.
I have watched good videos on how to color clay with oxides etc on the website CermicDaily.org It is free and there are lots of teaching video segments. Most are taken from videos they have for sale, but are great for starting research and deciding which you may wish to eventually purchase, or a book by the same instructor. There is another site called Clay Demo.com too. On the first site I believe it is Mitch Lyons that demonstrates making inlays of different colors by mixing in an oxide or perhaps it was cobalt.? There a teaching segments on using milfori techniques as well. Hope this is helpful.
Susan in Ohio