Both Uses of

contrast

in

Atonement, by Ian McEwan

Their sister, who sat between them, with left leg balanced on right knee, was, by contrast, perfectly composed, having liberally applied perfume and changed into a green gingham frock to offset her coloring.†
Chpt 1

by contrast = in a comparison that shows differences

She, by contrast, was leaning into his side, nestling her head on his shoulder and holding on to his shirt at the elbow awkwardly with both hands.†
Chpt 1 *

Definitions:

(1) (contrast as in: contrast their writing styles)
point to differences between; or compare to show differences
(2) (contrast as in: there is a contrast)
a difference -- especially a notable difference; or the side-x-side arrangement of things that draws attention to an unmissable difference
(3) (contrast as in: sharpen the picture contrast)
the difference between tones of an image -- as in a photo or video -- such as the quality of brightness or the intensity of shades or colors
(4) (meaning too rare to warrant focus)