The method of adding colour to a black and white photograph is referred to as Hand-colouring generally either to heighten the realism of the photograph or for artistic purposes. Hand-colouring is also known as hand painting or overpainting. Usually,watercolours, oils, crayons or pastels, and other paints or dyes are applied to the image surface using brushes, fingers, cotton swabs or airbrushes. Hand-coloured photographs were most popular in the mid- to late-19th century before the invention of colour photography and some firms specialized in producing hand-coloured photographs. 

These are a few examples of hand-colouring of photographs.

I particularly like this photography because it is neatly done and there are different shades of colour on the dresses, It looks realistic.

Before

After