Water pump?

If this leaks coolant externally only when it is cold then that sounds like a connection point that is loose and as it gets hot, the heat expands it enough to seal the leak. However, this is only going to get worse so we need to find the location.

Here are a couple guides that will help with this:

https://www.2carpros.com/articles/coolant-leak-symptoms

https://www.2carpros.com/articles/radiator-pressure-test

https://www.2carpros.com/articles/car-is-leaking-coolant

Once we know what is leaking then we will have a better idea what to replace but the most likely cause of something like this would be a heater hose.

So I am attaching the info from the manual on the cooling system so that we can inspect these items for leaks.

Please run through this info and let us know what you find.

Thanks