How to modify the render output of individual layers in Unity 2D?

So if I understand correctly, the way Unity 2D renders an image is to draw layer 1, then draw layer 2 on top of that, then drawing layer 3 on top of that and so on until every layer has been rendered. And so as a frame is being processed, there must be some point where Unity has rendered, say, layers 1 and 2 but not layer 3.

Is there any way to modify the current render before Unity starts rendering the next layer? Like can I write code that basically says "hey, when you're done rendering this layer, do something with the image before rendering the other layers", and if so then how?