Shaders - How to have multiple objects show different textures when it has some light.

I am currently working on a game where our art style needs to have a shader that will have a base black texture with some highlights that artists create, but when an area of the object is lit by a spot light/area light, it will show a different (lighter) texture that is being used with the object. What would be the best course of action for this shader? I would prefer not having to buy Shader Forge if possible. I am fairly new to writing code for shaders so I am not an expert on it :P

I appreciate any feedback.

Thank you,

Ben