It's probably due to grass that was better able to handle the shadier areas under those trees before you had them removed. Fine fescues are generally lighter, and handle shade very well. They are very fine, almost hairlike. Poa trivialis is an invasive light green grass that does well in shade. It spreads via stolons and is very difficult to remove. It usually fades out in the summer, only to come back strong in fall, and spring. A thatch rake can remove a lot of poa triv since it is stoloniferous and has shallow roots. It can also be susceptible to weekley applications of 2,4 D, but it's no guarantee.

This sounds like a micro nutrient issue. The trees sucked up some nutrient like iron, copper, or magnesium where they once were. Best way to find out is soil samples, but if you wanted to guess try iron.

Should I apply ironite only where I see the shade differences or over the entire lawn?

You should first determine if you have different types of grass in the lighter areas. Compare the lighter grass where the trees were with the darker grass in your lawn.

Hi, my next-door neighbor's lawn to the east-southeast of us had a big ole beautiful tree in his front yard up until about 3 years ago, when he had it chopped down and stump-grinded. That tree must have been at least 40 years old and possibly more if it existed here before the neighborhood/subdivision was built, but it probably came with the neighborhood. That tree used to drop lots of leaves into our roof gutter though.

The annual and perennial rye from the "Canada Green" grass seeds he planted where the tree used to be sprouted and grew but came in very light, a shade much lighter than the surrounding annual and perennial rye.

(By the way, he got very, very little Kentucky bluegrass and creeping red fescue from that bag of "Canada Green" seeded here in the St. Louis transition zone--and he seeded 2 bags from that company over 2 different seasons. It never did work out for him. What rye he did get growing died when temperatures climbed, just leaving the rye's thatch behind with precious little bluegrass and fine fescue to show for it.)

Anyway, now he doesn't have any grass where the tree used to be--just spurge and crabgrass, and a lighter shade of spurge at that. Whatever micronutrient deficiency plagued the rye there is now affecting the spurge.

If you want to get a good soil test (two actually) send them to Logan Labs in Ohio. They are excellent at getting the micros right.

Take several samples from where the trees were and several from where the trees were not. Keep them separated and send them in together with a full explanation of what you think is the situation. Then report your results back here. I would not rely on Penn State or your local county agent to provide as good a test as LL can for $20.