We generally tend to add more contrast in a photograph but its always great to have the background of the same tone in a picture many a times. In the photographs of the models, the face and the skin stands out more compared to the outfit as the background is also of the same colour. And thats exactly the intent, to draw the eye towards the subject rather than the outfit or the background.Red, a strong color makes the photograph bright and attractive without taking away the attention from the Girl in the photograph. In the other photographs also, the background being of the same tone leads the viewer to the face.

The shadow creates contrast and helps eccentuate the shape of the subject in the photograph which other wise has no contrast due to the background of the same tone. 

Here its the red that draws the eye first, compared to the face.

The colors do not always have to be very strong. Same Hue all though out gives a very soft look to this close photograph of the petals. Adding more contrast in an editing software would not be very desirable as it would be against the soft gentle feel which is being conveyed right now. 

Color and Contrast low