VLCD's work on a process of mild ketosis, which means it uses up its store of carbs and then starts to burn fat. Because of the low amount of carbs in a vlcd your body burns the fat off instead of the carbs off.
The atkins diet is not a VLCD and also works on a ketogenic diet, eliminating nearly all carbs, therefore doing the fat burning process.
The reason I mention the two is becuase on one diet food is had, and on a VLCD no food is had but they work on a similar principle, however atkins can also be high in saturate fats.
Logically, if you are showing pink then you are in Ketosis, if you have eaten a low/no carb food then you may well be, however it may take a few hours for the carb store to come back so the pink may not show on the next testing.
Im not a scientific person, just tried to explain the logic behind it, so if anyone has anything to add please feel free