This is an expression that we normally use when someone asks us how we are doing. When you say that you are in the pink of health, what you mean is that you are in good health; you are doing just great.
According to some scholars, the expression has nothing to do with the colour. “Pink”, I understand, is the name used to refer to the flower, “dianthus”. It is considered by many to be the perfect flower. Hence, when you say you are in the pink, what you mean is that you, like the flower, are perfect. Others believe that the pink refers to the scarlet coloured jackets that hunters wore when they went fox hunting. These jackets were called “pinks”. Therefore when you said that you were in the pink, it meant that you were in good health, ready to go hunting. Another theory is connected with the complexion of Caucasians. Being white, when they are in good health their cheeks are pink in complexion. Therefore, when someone says that he is in the pink of health you can actually see it on the cheeks. Since the cheeks of most Indians don’t turn pink, perhaps we should say, “We are in the brown of health”.