After some days exposure to light, all red cabbage + baking soda dye samples faded badly. The fabric scrap which I left unwashed turned beautiful green (it was blue before). But the fabric scrap which I washed in lukewarm water faded to grey. The scrap at the bottom was just some minutes in the dye solution (it’s also washed in lukewarm water): it’s bluish grey.
I cut up the washed fabric scrap (in the middle) to test three modifiers: a weak solution of baking soda, baking soda & vinegar, and vinegar. Baking soda turned the fabric pinkish grey. Baking soda & vinegar and vinegar, both modifiers helped to preserve a light green color.