How to darken one or more sprites with another sprite?

I have a sprites/GUI images and I want an overlayed image to darken only the sprites/GUI images, not anything else in the scene. Like multiplying the color of the sprite with (1 - n * alpha of overlay), where n is how much I want to darken it by?

You need a mask. But built-in masks’re used only for UI elements (Image and Text). Unity doesn’t provides masks for spriteRenderer (or other renderers) but you can find solutions based on custom shaders.