Considering that it doesn't look like your game calls the battle function (as tint does affect the battle function. I actually had a game where one of the battle mechanics was related to your tint), it actually shouldn't take too much work to implement the basic portion of it.
If you don't use tint in your game, some relatively easy ways to implement (just the tint portion, not any related mechanics) is have some events in your starting area that allow you to change the tint, add an item to your inventory that lets you change the tint, or a skill that lets you change the tint.
It is definitely true though that you might not have time to implement it depending on how your system is set up.