Currently I'm digging in the history of computer graphics and found the Windows default 20-color palette. It's based on the Windows and IBM OS/2 default 16-color palette but has the four additional colors Cream, Money Green, Sky Blue and Medium Grey. You can see both palettes in the Wikipedia. If you take a close look on the GUI and on icons back then from e.g. Windows 98 you'll see, that mostly the default 16-color palette gets used. Sometimes other colors were used as well, but not often. But I haven't found any example yet where the additional four colors are used. Does anyone has a clue what these colors are used for?
Those were used by the default desktop color scheme, I think starting in Windows 3.1 (it might have debuted in 3.0).
The “Sky Blue” color was the default Title Bar color; it’s unmistakable. The default scheme also used the “Cream” color for the Window Background.
“Medium Grey” was essential to create pleasant edges of the new “3D” buttons - I remember this specific fact being mentioned in a review/article at the time, probably in PC Magazine. Good looking 3D buttons required three shades of gray (plus Black and White of course).
I don’t remember for sure any specific use for “Money Green” but I remember liking it a lot. I think there was an additional desktop color scheme that used it in place of Sky Blue for the Title Bar.
I suspect the colors were picked because they were softer variants of the more intense colors of the original 16-color palette, being a natural extension. As I recall they could also be composed effectively via dithering in lesser (16-color) video modes.
It’s important to keep in mind that these colors were not the only colors available. The “20-color system palette” was defined in the context of VGA palette modes, typically 256 colors out of the 18-bit color (262,144 colors) full gamut. In VGA palette modes each pixel’s color is specified with an 8-bit color index in video memory, and each index would be mapped to an actual 18-bit color by programming the VGA palette registers. The “20-color system palette” were 20 color indexes reserved by Windows, fixed and always available. The UI would be rendered using those colors. Windows applications were allowed to ask to redefine any of the remaining 236 color indexes. Windows would flip around the VGA palette as necessary when each such application took focus. Drawing the UI with a set of fixed and guaranteed colors prevented a program changing the palette from turning the system UI into an unusable mess. And programs using only the system palette colors were guaranteed to look correct no matter what the current configuration of the palette was.
I remember the default desktop color scheme being quite pleasant, modern and easy on the eyes. it still feels that way.
Here’s an example: