How to modify Unity´s lighting prePass?

I wrote a toon shader, but I noticed that when multiple lights are affecting one object, they intersect and add to each other (which is bad, because I want the object to show the exact colors I choose in its shader). Here's a visual representation of the issue: https://gyazo.com/805c27186c1b7058b5e0699e809904a1

I have been reading a lot about it, and the only solution I found was to modify Unity's internal shaders, as explained here (at the end): http://docs.unity3d.com/Manual/RenderTech-DeferredLighting.html So I downloaded the files, put the shader in Assets/Resources, and assigned it as a custom shader from projectSettings/Graphics. Then I tried to modify the code. I didn't really know what to change, but I tried deleting all the content inside the lighting function, and there was no apparent change in my scene.

Am I doing something wrong? Is there (hopefully) a simpler solution? The effect looks quite simple. I'm not sure if I'm approaching it the right way.

Thank you

EDIT:

I also discovered "BlendOp", recently. Right now I'm using "BlendOp Max", and it gives me a close result to what I want, but there are still some artifacts: https://gyazo.com/c54d0706b308158d31c8b6b95706ed2b

What I would ideally want would be something more like this: https://gyazo.com/ca9d6e72fd7e4d2fe674aa2bf6541cea But I'm not even sure it's possible.

Any help or directions would be highly appreciated.