The 0x1000000 bit (NH_BASIC_COLOR bit) was used to mark
CLR_BLACK when storing it in u->ucolor. Now, all of the basic CLR_*
colors are stored that way.
The NH_BASIC_COLOR bit indicates that the value in u->ucolor is
not an rgb value, rather it is one of the 0-15 basic NetHack colors.
The window-ports need to strip the NH_BASIC_COLOR bit off before using
it for color changes.
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