int use_default_colors(void);
int assume_default_colors(int fg, int bg);
Applications that paint a colored background over the whole screen do not take advantage of SGR 39 and SGR 49. Some applications are designed to work with the default background, using colors only for text. For example, there are several implementations of the ls program which use colors to denote different file types or permissions. These "color ls" programs do not necessarily modify the background color, typically using only the setaf terminfo capability to set the foreground color. Full-screen applications that use default colors can achieve similar visual effects.
The first function, use_default_colors() tells the curses library to assign terminal default foreground/background colors to color number -1. So init_pair(x,COLOR_RED,-1) will initialize pair x as red on default background and init_pair(x,-1,COLOR_BLUE) will initialize pair x as default foreground on blue.
The other, assume_default_colors() is a refinement which tells which colors to paint for color pair 0. This function recognizes a special color number -1, which denotes the default terminal color.
The following are equivalent:
These are ncurses extensions. For other curses implementations, color number -1 does not mean anything, just as for ncurses before a successful call of use_default_colors() or assume_default_colors().
Other curses implementations do not allow an application to modify color pair 0. They assume that the background is COLOR_BLACK, but do not ensure that the color pair 0 is painted to match the assumption. If your application does not use either use_default_colors() or assume_default_colors() ncurses will paint a white foreground (text) with black background for color pair 0.