Yeah there are typically a "Cold" line and a "Hot" line on the coolant overflow tank if you just want to visually see that.
Not on my XK just one line. I just had the thermostat housing replaced a week or two ago and want to top things to the line at the right time to insure I still do not have a leak. I am well aware of pressurized system, cutting my may limited mechanical skill as a gas pump jockey in the mid 60's. Yes there was a time only the real MACHO's pumped their own gas. Now NJ is the only place it may still happen that I know of.
My XK has "Full" and "Add" lines.
And often cold its at the "Add" line, but at the "Full" line when warm.
Don't get wrapped about it, think about it, its an "Overflow" Tank.
The proper level of coolant is the pressure side of the coolant totally filled with coolant and no air in it. The overflow tank holds extra that is forced out from the pressure side of the system when the fluid expands as it gets warmer, so it can then suck it back in when the coolant cools and contracts, without sucking air into the system.
So all you really need in the overflow tank is that amount that is forced out/sucked in as the coolant expands/contracts. Plus a little extra in case there is a slow leak.
The actual level has little effect on the cooling system, as long as its as its enough.
Know what happens if you overfill it? Perhaps when the fluid expands, the level might go up to the point it spills over the vent in the bottle and an ounce or two spills on the ground. Know what happens if its underfilled, nothing, if its way low, perhaps you'll suck a few onces of air into the cooling system, which it can handle just fine.
The other systems, engine oil, transmission, axles, over filling or under filling has big consequences. NOT the coolant overflow bottle, it won't make much of difference one way or the other.
What will make a difference with the cooling system, is if there is a slow leak you don't notice, you'll slowly loose coolant and very likely won't show up on the level on the overflow bottle. That is why you need to open the pressure cap (only when the engine is cool) to check the cooling system is filled to the top (or within an inch) under the pressure cap. If its not, then you have a leak, and if its a slow leak you can't find, you need to keep doing this, to keep the system topped off.