You get a list of defined color names in 'xwi' with:
attributes.colors.setlist("xwi")
Nice, it works! I note the first method only works when \usecolors[xwi] is present; otherwise, it prints an interesting "themaintextcolor".
Here is a nostalgic mkii accessor for such a list (color support is a
real old feature and conceptually hasn't changed that much):
\def\WhateverActionIWant#1{#1\par}
\settrue\collectcolorsinlist \setupcolor[xwi]
\processcommacommand[\colorlist]\WhateverActionIWant
but you probably don't want to catch up on that.
It doesn't work in Mark IV (\colorlist is undefined). Maybe I'm too young, but Mark II was already frozen when I was a teenager, so I'm not sure I want to try it now. Thanks, Hans!
Cordially,
Jairo :)