216 colour cube
“The Netscape palette for foreground colors usually (but NOT always) consists of all the combinations of 00, 33, 66, 99, CC, FF for each of the red, green, and blue elements of the color descriptor. This results in 216 (6×6x6) distinct colors.” – Victor Engel
A colour taken from the 216 cube is guranteed to reproduce as solid colour without dithering on most Web browser and screen combinations. Page colours and menu items are usually taken from this colour space.