Somber and sombre are different spellings of the same word, meaning (1) dark and gloomy, or (2) melancholy. Somber is preferred in American English, while sombre is preferred in all other major varieties of English.
Examples
For example, these non-U.S. publications use sombre:
Joy was met with sombre remembrance as 42 fresh faces entered the Toronto Police Service Thursday. [Toronto Sun]
Derek Cianfrance’s film is a sombre, painful portrait of a toxic marriage … [The Guardian]
Jazzing up subject titles may help revive interest in Indonesian studies, an otherwise sombre paper on the decline of the discipline reports. [The Australian]
And these American publications use somber:
Those events complete a somber week at the White House… [Washington Post]
Forgive the somber tones this week, but February is always a tough month for me. [Wilkes Barre Times-Leader]
Everything he touches comes out sounding somber and a bit grandiose. [San Francisco Chronicle]
