Question
Usually in the fall I get patches of a lime green mixed in with the green grass.  Do I need sulfur or iron?  The soil is very acid.

Answer
If the soil  is very acid, you do not need sulfur. If the soil is very acid the grass may suffer from iron toxicity which is more a browning/rusting appearance which you will not be able to rub off because it is coming from inside the leaf.
My considered opinion is that you have a different problem.