My elm has recently begun to develop some darker leaf edges. At the same time some new leaves appear yellowed.
I bought the tree a couple of months ago and I fertilized it twice 15 days apart with a solid granular fertilizer (from next month I will use a liquid one which I think is better). At the same time I gave him micronutrients to supplement and use a pesticide spray for prevention. I water it when I see the earth starting to dry out, basically once a day/every two days.
There are many new buds and it is growing well, the problem is this unusual color of the new leaves. The old, already dark green leaves have remained unchanged.
This has been happening for a few days, a week at the most.

Below I attach photos in progression: the first of March 25, the second of March 31 and the third, fourth and fifth of today.

What can it be? Thanks in advance for your answers.

Using a pesticide for no reason is not a good idea, I can't see anything wrong with this plant, as long as it is outside you seem to be doing most things right. If it is not broke don't fix it as the saying goes. The new leaves are fine, that is how they are when they start to grow.

My chinese elms new growth is always light green, not redish as yours.

Basically there are two possible reasons for redish new growth. One beeing a lot more sun and the other is lack of nitrogen. Since you fertilized before the growth started I would not think of nitrogen deficiency as part of the explanation.

Not that I see any problem with your tree. But If you don't like the look of the red new growth you can move it to a location with a little bit less sun. I would not.

Forgive my not totally correct English, but it's not my language. I wrote pesticide but I meant a product that keeps insects and parasites away. in fact I used Neem oil, nothing else.

Could this be a symptom of overfeeding? the indications on the solid fertilizer were not precise, also for this reason I decided to use a liquid one from next month.

No, over fertilising is difficult to do. Looks fine that is the colour they are when they emerge. It is early to be adding fertiliser, but it won't hurt.