Can I use amaranth flour or another grain flour as a substitute for white flour in tortillas?
You might try to add some amaranth flour or any other kind of flour, but it's difficult -impossible I think- you can make a bread or a tortilla out of 100% amaranth flour. Wheat flour (that you call "all purpose white flour") contains gluten, that is functionally used to make dough stick together. If you use amaranth, rice or other kinds of flour you'll find that it's difficult to make the dough.

If you want to make a healthier tortilla use whole wheat flour, and add whatever you want inside: amaranth, sesame seeds, linseed, nuts, peanuts, almonds, etc.