Synonyms for Somber (same or very similar meaning)
WordNet sense 1 (lacking brightness or color; dull):
drab, sober, sombreWordNet sense 2 (weak in color; not colorful):
colorless, colourlessWordNet sense 3 (grave or even gloomy in character):
melancholyWordNet sense 4 (lacking cheer; depressing):
uncheerfulFrom the ODE community, based on WordNetadd/edit