In restoration, retouching a painting describes the process where paint is applied to areas where paint is missing.
The original is not touched and nothing is improved or altered.
The paint medium and pigment has to be stabile but nevertheless reversible.

Here some of the most common types of retouching – “from minimal or no integration to re-creation of lost content … “*
Neutral Retouching: Only a base coat, the imprimatur, the primer is applied
Tonal Retouching: A slightly paler tone of the original is applied
Abstract Retouching:  Tratteggio –  a selective color separation technique which was developed during the restoration of  “The Cross of Cimabue”
 Total Integration Retouching: 
This is most commonly used technique and matches the color and tone precisely.

Which technique of retouching to apply to what specific job can become a matter of great debate.

Getting the right tone…
The colour tone gets physically mixed on a palette but first in the mind of the „retoucheur or the retoucheuse“.
It is quite a meditative process and an interesting journey – one millimetre at a time.

Most colours that we perceive as one tone – green, yellow, blue, red  –  are always a combination, a mix of colours.
On a magnified level a pale green can reveal itself as a combination of blue and yellow, speckled with small off white dots, that are interrupted by thin brown lines.
To master the skill of retouching takes many years of practice.