The Netscape standard colour palette consists of a 216 colours. Each of the red, green and blue components is of one of six levels (0,51,102,153,204,255 or Hex-00, Hex-33, Hex-66, Hex-99, Hex-CC Hex-FF) giving a 6x6x6 colour cube. The left block contains the 'Strong' 152 colours which are on the outside of the cube. The middle block is the next layer within the cube and contains 56 'Medium' colours. The third block has the innermost 'Weak' 8 colours. Colours on the same row have equal luminance. The strong colour block contains the most saturated colours.
The right hand set of 'Grey' levels is NOT part of the standard Netscape standard palette, but it is often useful to have these sixteen levels for monochrome applications. The grey levels are Hex-00, Hex-11, Hex-22, Hex-33 etc.. up to Hex-FF. They are coded in the GIF using some of the remaining 40(i.e. 256-216) colour positions. You browser may however dither a number of these grey levels on your display.
The area between the colour blocks is yet another grey level of Hex-80, the true 'mid-grey'. Your browser may show this as transparent or white.
This page and the image are '© 1996, Alan S. Murphy'. However, you are welcome to save the GIF and use it as your palette in a paint program. The only condition is that you use the GIF 'as is' and do not alter it in any way. You may, for example, open it as an extra image in Paint Shop Pro and use it to fill the dropper.
You can find more information on colour palettes at Victor Engel's No Dither Color Palette and at Sun Guide to Web Style - Technote.