A Win32 console application can make use of only 16 colors. And an
application does not request a specific color, but request an index in
the color table. You can apply any color mapping you want, but an
application will expect that the color mapping corresponds to something
usable: for instance, it can request a darkBlue background and a
lightGray foreground, but if you use a mapping where both are red, you
won't see anything.
For octave, you can tune QTerminalColors.cpp to whatever you like,
octave itself only makes use of 2 colors anyway.