Modesty is to merit as shadows are to the figure in a painting: it strengthens it and sets it off.
 
[La modestie est au mérite ce que les ombres sont aux figures dans un tableau: elle lui donne de la force et du relief.]

Jean de La Bruyère (1645-1696) French essayist, moralist

The Characters [Les Caractères], ch. 2 “Of Personal Merit [Du Mérite Personnel],” § 17 (2.17) (1688) [tr. Stewart (1970)]
    (Source)

(Source (French)). Alternate translations:

Modesty is to Merit, what Shades are to the Figures in a Picture; it gives it Strength and Heightening.
[Curll ed. (1713)]

Modesty is to Merit as Shades to Figures in a Picture; giving it Strength and Beauty.
[Browne ed. (1752)]

Modesty is to merit what shade is to figures in a picture; it gives it strength and makes it stand out.
[tr. Van Laun (1885)]