How to make sprites become darker when in shadow?

I have 2d sprites in 3d environment, and I’d like them to get darker the more they are engulfed in shadows. The left image below is how it looks currently and the right one is how I’d like it to look. I’ve tried fiddling with shaders and whatnots but without a result.

So, is this possible to have sprite become darker when it is positioned in a shadow?

switch the sprite to darker one, does not matter if it’s pregenerated in photoshop or in unity … it’s so far the easiest way, but if you need many sprites it’s doubling the number of pixels so it might affect performance

or you can use material with shader which will be affected by color and then you can just set the color of material to affect lightness of the bitmap