This is the "A Girl" by Jean-Baptiste Greuze. Painted in the French style, it's a beautiful example of neoclassical portraiture from the 18th century. Look at her transparent shawl, or afichu - it covers one shoulder but has fallen off the other. Her white muslin dress is only loosely held up over her chest, an effect enhanced by her bodice having come undone.
The fashion in France at the time was for such dresses and for clothes to be worn 'naturally'. You can see this reflected in the way Greuze paints the girl's clothing. The palette is dominated by shades of white and grey, but it's warmed up by the rose shades of her cheeks, lips, and ear.
Notice how the girl's right arm is raised as if reaching for something, while her left arm is by her side. Her expressionless face and upward gaze to the left of the picture are characteristic of Greuze's style. In fact, this format was repeated in many of his single female figures.
A Girl hangs in The National Gallery on Trafalgar Square, London — free to enter. Point your phone at any artwork there and audioguide.london plays a free audio guide in six languages — no app download needed.