Custom (Sprite-)Shader: Combine Sprite with light and original Sprite

I recently started using light in my 2D game and thus using the shader Sprites/Diffuse (instead of Sprites/Default) for all my sprites.

However, I would like to kind of "limit" the amount of light, a sprite can maximally gain to prevent details from "burning out" in white. (I would not set a hard limit, but probably try to find a mathematical function to work similar to a tone curve in Photoshop/image editing)

I assume, writing a custom shader is the way to go here; I don't have any experience with shader programming yet, therefore my question: Is there any point in the rendering pipeline where I can access two things,

1. The RGB values of the sprite as determined by all the lighting currently on the sprite (using the Lambert model as Sprites/Diffuse appearantly does)

2. The original RGB values of the sprite, as if there were no lighting at all

and then return RGB-Values in which I actually want the sprites (or each pixel; if I understand shaders correctly, they are calculating the RGB values for each individual pixel) to be rendered with?

Is there a way to achieve that and could someone point me in the right direction how I could create a shader that basically works like Sprites/Diffuse but allows me "access" like described above?

Thanks for any advice!