Advice on a Saturation shader

Hello everyone, happy new year! I'm looking for some advice on a shader using Unity, I havn't scripted shaders other than changing Material properties & Unity Shader graph shaders..

but i'd like to begin studying more into it all as i've found a keen interest in them..

what i'm after specifically is a shader(or even an image effect) that adds colour to the environment, based on where the player is currently, has previously been and has interacted with.

some things i've thought about were having a radius around the character that changes the saturation based on how close an object is to the player, although it would only work on objects and not terrain.
some other things i've looked at are
https://assetstore.unity.com/packages/vfx/shaders/fullscreen-camera-effects/gray-scale-pro-64686

https://assetstore.unity.com/packages/vfx/shaders/advanced-dissolve-111598

that could help as references, but i don't believe they'll do what i need them to do
thank you for any resources/advice that can be given