Holy cow, I wrote a book!
There are six styles available to the static controls which draw
frames and rectangles in one of three fixed colors:
But if you actually create a static control with one of these styles,
you'll find that the color isn't actually black, gray, or white.
So why are they called black, gray, and white?
Because they used to be black, gray, and white.
Knowledge Base article 125684
gives the history behind these styles.
Back in the 16-bit days, these styles really did give you
black, gray, and white, or at least they did if you used
the default color scheme.
Windows 95 shifted from using the window colors
to the 3D colors,
presumably to give the system a less harsh look.