What is meant by "Pink of Health"

"Pink of Health" means best of the health. When some one ask how are you doing and you say I am in the pink of health then it means that you are in good health and are doing great. Pink of health denotes one's good health in physically as well as mentally.
Believes for this saying:
  • When one is healthy, the blood circulation is quite noticeable and this results in redness of face. This is what is meant by 'in the pink of health'.
  • It means you have a rosy glow. It means you look healthy, that you don't have a sickly pallor, which can be a sign of illness.
  • “Pink” , 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. 
  • 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”.

Origin of the saying:

The earliest citations of 'in the pink' are from the 16th century and, at that time, the meaning was 'the very pinnacle of something', but not necessarily limited to health. The earliest example that I can find of pink being used with that meaning is from 1597 Shakespeare's Romeo & Juliet, 1597:
Mercurio: Why, I am the very pinke of curtesie.
The earliest citation I've found for 'in the pink' is from Leigh's Kensington Gardens, 1720:
"'Tis the Pink of the Mode, to marry at first Sight: - And some, indeed, marry without any Sight at all."
The 'pink of the mode', that is, the acme of excellence of fashion, was a common early usage and continued to be used throughout the 19th century, as here from Thackeray, 1840:
"In the very pink of the mode."
Dickens, in his Letters, 1845, demonstrates the difference from our current usage:
"Of all the picturesque abominations in the World, commend me to Fondi. It is the very pink of hideousness and squalid misery."
It isn't until the 20th century that we find the phrase in the 'pink of condition' form that is currently used, in the Kynoch Journal, 1905:
"Makers may despatch explosives from the factory in the pink of condition."