I love that color too. Fun trivia that I discovered doing pixel art way back then: That color is blue.
It appears purplish, but it's actually a desaturated blue with hue right at 240°. Something about the lack of saturation and brightness gives it a purplish cast.
It’s funny how mixing pure blue (#0000ff) with pure white (#ffffff) makes it look a little purple, and how you also have the exact same problem when you invert the colors - mixing pure yellow (#ffff00) with pure black (#000000) makes it look sort of green (the complementary color to purple). In both cases you have to adjust the green channel a little “faster” and the red a little “slower” to get something that looks more neutral, ie shift the light blue towards cyan and the dark yellow towards red.
That seems similar to the "perwinkle blue" that was used as the default background in Apple IIGS ProDOS16 / GS/OS. In that case it was done with pairing white pixels with blue pixels.
None of the alternate themes actually made it into a final release of Mac OS; just Platinum.
There was a fairly healthy third-party theming community, though, and the Apple-developed themes (Memphis, High-Tech, and a sketch-styled theme called Drawing Board) would still work if you got your hands on them.
Drawing Board was probably the most usable of Apple's alternative Appearance themes (which never shipped, but were leaked pretty quickly). The little lines extending off the corners of windows had some glitchy behavior in some apps, IIRC, but it was at least visually acceptable, and I used it for a while.
The other two themes were basically unusable, though, and it's very clear why they were never officially released.
It appears purplish, but it's actually a desaturated blue with hue right at 240°. Something about the lack of saturation and brightness gives it a purplish cast.