WordNet sense 1 (a dull greyish to yellowish or light olive brown):

olive drab

WordNet sense 2 (lacking in liveliness or charm or surprise):

dreary

WordNet sense 3 (lacking in liveliness or animation):

dull

WordNet sense 4 (lacking brightness or color; dull):

sober, somber, sombre

WordNet sense 5 (weak in color; not colorful):

colourless

WordNet sense 6 (of a light brownish green color):

olive-drab

WordNet sense 7 (being or having or characterized by hue):

chromatic

WordNet sense 8 (causing dejection):

blue, dark, depressing, dingy, disconsolate, dismal, drear, gloomy, grim, sorry, dispiriting, dingy

WordNet sense 9 (lacking cheer; depressing):

uncheerful