Ah, I see what you mean now, black tank vs shiny tank, makes sense.
I keep my over flow tank about a 1/3rd full and my surge tank about 1/2 full. The overflow typically fills up almost to the top when the engine gets to full temp following a good drive. Then as it cools that water is drawn back into the engine, works out real nice. On occasion I've put to much water in the overflow and then it filled and overflowed.
Before I ran an overflow tank I would have to add water to the radiator on occasion as it would eventually get to low from the water being pushed out and lost.
My surge tank hose goes into the bottom of my overflow tank, but some use a hose that goes in the top and then reaches down to the bottom of the tank inside.
An overflow tank is required at some race tracks, as are various "puke" tanks, all in an effort to keep fluids off the race track.