The onyx colour is named after stone of the same name - average value. This stone with white veins is a color used for black backgrounds.
#F6F6F6
#E7E7E7
#D1D1D1
#B0B0B0
#888888
#6D6D6D
#5D5D5D
#4F4F4F
#454545
#3D3D3D
#0F0F0F