1.  sombre(s = adj.all) drab, sober, somber - lacking brightness or color; dull; "drab faded curtains"; "sober Puritan grey"; "children in somber brown clothes"
Derived form noun sombreness₁

2.  sombre(s = adj.all) melancholy, somber - grave or even gloomy in character; "solemn and mournful music"; "a suit of somber black"; "a somber mood"
Derived forms noun sombreness₃, noun sombreness₂

1.  Dull; dusky; somewhat dark; gloomy; as, a somber forest; a somber house. [1913 Webster]

2.  Melancholy; sad; grave; depressing; as, a somber person; somber reflections. [1913 Webster]
"The dinner was silent and somber; happily it was also short." [1913 Webster]

   To make somber, or dark; to make shady. [1913 Webster]

   Gloom; obscurity; duskiness; somberness. [1913 Webster]

sombre, adj. (also US somber)
1 dark, gloomy (a sombre sky).
2 oppressively solemn or sober.
3 dismal, foreboding (a sombre prospect).

Derivative:
sombrely adv. sombreness n.

Etymology:
F sombre f. OF sombre (n.) ult. f. L SUB- + umbra shade