On Windows, the existing colors before Netscape starts are remapped to the Netscape palette. Sometimes, when you finish with Netscape, or minimize it, the previous palette will not be restored causing your display to appear with funky colors. To fix this, restore and then minimize Program Manager. This will work for other programs that change the palette as well.
On Macs, the additional 40 colors are also used by Netscape.
On unix workstations, from what I have been able to glean, if more than 40 colors are already in use, Netscape will not be able to allocate 216 colors, so it sets up a 5x5x5 (125) color cube instead. It might even resort to using a 4x4x4 (64) color cube if more than 131 colors are already being used. Netscape color grid is a page in Berlin showing these cubes with hex values.
On the other hand, 12 bit systems may use an even larger palette. Netscape uses a 16x16x16 color cube on these systems. Unfortunately, as with the palettes used on Netscape for X Windows, most of the colors do not coincide with the colors in the 6x6x6 color cube.
On 15 bit systems, a 32x32x32 color cube may be used. On 16 bit systems, I think a 16x32x16 color cube is used, but I still have to do more research to verify this.