How to implement "layers" to a 2d tower defense game (c#)?

Questions & Answers
1

What I mean by this is take Bloons tower defense, for example. When the tower pops a balloon it goes to the next layer. Ex: If a tower pops a green balloon, it turns into a blue balloon, then if that is popped, it turns into a red balloon, then it will finally be popped.
Here is an example: Bloons Tower Defense 5 - Gameplay commentary - YouTube

I was thinking making a class or array for balloon, but i’m not sure how I can transform one balloon into another, and it is still a balloon class
Any advice would be greatly appreciated.

2

Just do something like

if(Dart == Balloon)
{
GreenLayer.SetActive (false);
BlueLayer.SetActive(true);
}