Rain Barrell in the winter...paint it black, insulate it, or insulate and paint that black?

[Cattle dog Mom]

Thanks. Yeah, I'm still thinking I want a tank heater and circulating pump as a backup in case things get close to freezing, but I'm looking for the most efficient setup that would allow the heater/pump to be used the least often.

I would just put a stock tank heater in it and forget about it. (Meaning forget about the sun keeping it thawed or not. Stock tank heaters don't run 24/7, they only come on when the water reaches a certain temp, then turn off.) A pump is just going to use more electricity for no reason. I've used a stock tank heater in my horses stock tank for years with no issues. (I only use the sinking type, NOT the floating type. Just make sure it is made for plastic tanks and you will be fine.)