There should be a hand-operated valve in the line to the water heater. Turned all the way one way it shuts off the water to the heater. Turned the other way re-opens the heater line.
It won't hurt to leave the pink stuff in the HWT until spring, but it will need to be thoroughly flushed. My personal method of winterising is to use compressed air at about 30 pounds per square inch into the city water inlet and then open all the faucets, the shower, outside shower and toilet flush to blow all the lines clear of water.
Once clear, there's nothing left to freeze except the P-traps, so I pour about a cupful of antifreeze into the bathroom and kitchen sinks and down the shower drain.
Don't forget to drain the fresh-water tank.
We don't get hard freezes in this part of the country. It's rare for daytime highs to stay below freezing, so winterising isn't quite as critical for us.