Windows 95 users will find the appropriate settings in the 'display' section of control panels (look in 'Settings' on the start button if you're stuck), Windows 3.11 users can change their settings through either a third party utility which came with your graphics card, or the 'Setup' thingie in your control panels, though this latter is a dodgy option of you don't know what you're doing and you can end up with no display at all and a fair amount of win.ini file grief. Mac users should look in Control panels for the 'monitors' option.
If you're stuck with a 256 colour graphics card or monitor, try closing down any other applications, as Netscape deals with colours more and more badly as your machine runs low on resources. Internet Explorer 3 doesn't have a problem with 256 colour displays, so in extremis you might want to try changing your browser. I hope this helps.