Yellowing, from the bottom up, is totally normal. A lot (like half) of your fan leaves will yellow and die off, from the bottom up, while flowering. I'm not sure if that's what's happening with you or not. If a plant has been damaged by deficiency/toxicity, the damaged parts will remain damaged permanently. Meaning, you may have had a problem that flushing fixed, but those yellow leaves will never turn green again.
just make sure your new growth is looking good.
also, remember that nitrogen makes chlorophyll. Chlorophyll keeps everything green. Chlorophyll also makes everything taste bad. Leaves need nitrogen. Buds do not (or much less).