One trick that comes with shaders is using them to reassign colors to assets after the fact. This lets us create and re-use gray-scaled assets in a variety of places without needing to duplicate the assets to change up the colors.
I've seen people create the game completely in gray scale, and then pop a little color-reassignment shader on top.
BUT HOW!?
This is a fun little one, a shader I'd love to implement and share the learnings from.