How do I make a territory(something like paint on the ground) that hurts enemy's and where the surface of the territory can be calculated?

How do I make a territory(something like paint on the ground) that hurts enemy’s and where the surface of the territory can be calculated?

(I’m not that experienced)
I want to make a territory something like splatoon(The paint). The enemy’s must then take damage if they step into it. I also want the paint able to fade away after a while. I also don’t know how I can then calculate how much paint there is on the ground.

The thing is, I don’t know how my idea can be made. Is it something with landscape painting or should I do this different?

It would mean a lot to me if someone could tell me how I can make this or just point me in the right direction.