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Synonyms for Sombre (same or very similar meaning)

WordNet sense 1 (lacking brightness or color; dull):
drab, sober, somber

WordNet sense 2 (weak in color; not colorful):
colorless, colourless

WordNet sense 3 (grave or even gloomy in character):

WordNet sense 4 (lacking cheer; depressing):
cheerless, uncheerful

From the ODE community, based on WordNetadd/edit