The variegation in Canna ‘Stuttgart’ foliage is different from other forms of Canna variegation. It is caused by chlorosis, and in certain cases the plants have “whited” themselves to death, i.e. photosynthesis cannot take place at all.
This is also why the chlorotic foliage eventually turns brown; the tissue dies without the ability to photosynthesise which means that no water and its dissolved minerals is being drawn up to the leaf.
Attempts to stop the browning are doomed to fail, as this is just an unsupportable plant form.