I came across this very interesting article in one of the old in-flight magazines of India Airlines.
In the 19th century, photography came to India and rapidly replaced the traditional portrait paintings. Many painters switched professions and became artist-photographers. They would hand-colour the black & white pictures partially or sometimes completely to give it a realistic appearance.
This technique became very popular in the late 19th century as the final product usually revealed the artist photographers artistry and fine workmanship. Very true.
(images from Swagat, in-flight magazine. Nov,1999)