In cold conditions the flow is reduced, when it is warm the flow increases. Humans can do this too, which is why our hands and feet become white when we are cold and pink when warm. Control is very sophisticated and involves the hypothalamus and various nervous and hormonal systems. However, penguins also have 'countercurrent heat exchangers' at the top of the legs. Arteries supplying warm blood to the feet break up into many small vessels that are closely allied to similar numbers of venous vessels bringing cold blood back from the feet.
Heat flows from the warm blood to the cold blood, so little of it is carried down the feet.
Ducks and geese have similar arrangements in their feet, but if they are held indoors for weeks in warm conditions, and then released onto snow and ice, their feet may freeze to the ground, because their physiology has adapted to the warmth and this causes the blood flow to feet to be virtually cut off and their foot temperature falls below freezing. I cannot comment on the presence or absence of colateral circulation, but part of the answer to the penguin's cold feet problem has an intriguing biochemical explanation. Usually the same amount of heat is absorbed in the reverse reaction, when the oxygen is released by the haemoglobin. The actual value of delta H varies from species to species. In Antarctic penguins things are arranged so that in the cold peripheral tissues, including the feet, delta H is much smaller than in humans.