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.
The use_default_colors function was added to support ded. This is a full-screen application which uses curses to manage only part of the screen. The bottom portion of the screen, which is of adjustable size, is left uncolored to display the results from shell commands. The top portion of the screen colors filenames using a scheme like the "color ls" programs. Attempting to manage the background color of the screen for this application would give unsatisfactory results for a variety of reasons. This extension was devised after noting that color xterm (and similar programs) provides a background color which does not necessarily correspond to any of the ANSI colors. While a special terminfo entry could be constructed using nine colors, there was no mechanism provided within curses to account for the related orig_pair and back_color_erase capabilities.
The assume_default_colors function was added to solve a different problem: support for applications which would use environment variables and other configuration to bypass curses' notion of the terminal's default colors, setting specific values.