For an example; I want to create an overlay with some swatches, that only affects African elders
Is there any tutorial about this?

Well, the functionality described in the title is quite a different one to the example.

Forcing all Sims of age+gender to have a certain overlay would be as simple as editing that overlay onto the default skin for that age+gender.
While ethnicity is quite different, as I believe that only exists as a tag for CAS items and isn't actually something sims themselves can be defined as. These tags influence what is used during random generation, but they don't restrict usage.
You could edit the specific skintone swatches of your choice, however. But the overlay being separate and having multiple swatches is at odds with wanting it to be forced/a default replacement.

Depending on what it is you actually have in mind, your best bet would be looking into information about how overlays vs non-default and default skintones work in general.

Strictly speaking, you can create a buff that sets CAS_part to that overlay, inject that buff to trait_Elder using XML Injector, and set tests on _add_test_sets for gender if you want multiple versions. But there are no tests for ethnicity outside of perhaps the Filters that can spawn Sims, as far as I'm aware. You would have to somehow mark them manually in a manner that could be picked out by the buff's tests, or start overriding Filters, though the latter would only affect new Sims.

Thanks, as I thought, it's no easy(almost impossible)
Well I"ll keep fighting with the Sims4 Studio then