Rust color usually related to Cal, PH problems are actually very rare, however lockouts are very common
would need more info as to how you grow and pics of the plants ???
I'm hesitant to post picks, but will if necessary. 600w hps. Growing in sunshine advanced #4 (peat moss, which can be acidic) feeding gh maxibloom, calmag, and root enhancer with every watering at recommended amounts, with no signs of burn yet. I'm in week 3 of flower and they are looking fantastic.
A couple of my strains are getting a small amount of yellowing, which I suspect may be nitrogen def. But my mentor said that it was because they were too close to the light. They were about 8 inches away from an open reflector. So I moved it a few inches back, and now a slight rust color on a couple strains.
Edit: worth noting that the yellowing which started at the bottom and worked its way up usually on large fan leaves, were affected by the plants closest to the light. Meaning directly under it. As opposed to the plants slightly on the side