A little more specific information may be necessary.  If the nipple is
normal colour after the baby comes off the breast, but then turns white,
this is due to vasospasm (Raynaud's like phenomenon) of the nipple.  The
pain may radiate into the breast and is due to local ischemia due to
decreased blood flow.

If the nipple is white immediately the baby comes off the breast, I
don't know.

I am not convinced about mastitis. Without a lump, warmth, constant
pain, you can't make a diagnosis of mastitis.

Jack Newman, MD, FRCPC