The win9x default gray was from an age where you didn't necessarily have the option to use an arbitrary shade of gray - meanwhile Macs had full control of the hardware (as they still do) and could pick any old color.
That said, you _could_ customize all the colors in Win9x if you liked, which you couldn't do with Macs.
> That said, you _could_ customize all the colors in Win9x if you liked, which you couldn't do with Macs.
Well, until Kaleidoscope[0] was created way back in 1991, followed by Apple's own Appearance Manager[1] in Mac OS 8.5 in 1998, both of which allowed all-encompassing theming that could stop just short of changing colors or radically change your UI's appearance. Windows wouldn't get an equivalent until the addition of visual style themes in XP in 2001 (which still had to be hacked to allow arbitrary third party themes) and was by predated the third-party WindowBlinds[2] by three years.
To date I've not seen OS theming that was as capable as Kaleidoscope was. Kaleidoscope schemes could change practically any aspect of the Mac OS UI (e.g. moving titlebars to the bottom or left/right edges of windows) which led to the creation of some extremely creative and zany schemes. Even the theming systems of modern Linux desktop environments which are built with customization as a central tenant aren't as capable as Kaleidoscope was.
That said, you _could_ customize all the colors in Win9x if you liked, which you couldn't do with Macs.