Serious, sad, and without humour or entertainment. (Cambridge Dictionary)
Dark and plain. (Cambridge Dictionary)
Dark or dull in colour or tone. (Oxford Dictionaries)
Having or conveying a feeling of deep seriousness and sadness. (Oxford Dictionaries)
If someone is sombre, they are serious or sad. (Collins Dictionary)
Sombre colours and places are dark and dull. (Collins Dictionary)
The spirited, mischievous and almost jovial first day of the Allies in the camp is counterpointed by the sombre, grim and grizzled looks of the Russian prisoners used as forced labour.
Source: Film/ The Great Escape.
The Grim Reaper, in his days as a frequent figure of folk tales, was originally used as a menacing, sombre symbol of the inevitability of death.
Source: The Grim Reaper