If a terminal is capable of redefining colors, the programmer can use the routine init_color to change the definition of a color. The routines has_colors and can_change_color return TRUE or FALSE, depending on whether the terminal has color capabilities and whether the programmer can change the colors. The routine color_content allows a programmer to extract the amounts of red, green, and blue components in an initialized color. The routine pair_content allows a programmer to find out how a given color-pair is currently defined.
The init_pair routine changes the definition of a color-pair. It takes three arguments: the number of the color-pair to be changed, the foreground color number, and the background color number. For portable applications:
If the color-pair was previously initialized, the screen is refreshed and all occurrences of that color-pair are changed to the new definition.
As an extension, ncurses allows you to set color pair 0 via the assume_default_colors routine, or to specify the use of default colors (color number -1) if you first invoke the use_default_colors routine.
The init_color routine changes the definition of a color. It takes four arguments: the number of the color to be changed followed by three RGB values (for the amounts of red, green, and blue components). The value of the first argument must be between 0 and COLORS. (See the section Colors for the default color index.) Each of the last three arguments must be a value between 0 and 1000. When init_color is used, all occurrences of that color on the screen immediately change to the new definition.
The has_colors routine requires no arguments. It returns TRUE if the terminal can manipulate colors; otherwise, it returns FALSE. This routine facilitates writing terminal-independent programs. For example, a programmer can use it to decide whether to use color or some other video attribute.
The can_change_color routine requires no arguments. It returns TRUE if the terminal supports colors and can change their definitions; other, it returns FALSE. This routine facilitates writing terminal-independent programs.
The color_content routine gives programmers a way to find the intensity of the red, green, and blue (RGB) components in a color. It requires four arguments: the color number, and three addresses of shorts for storing the information about the amounts of red, green, and blue components in the given color. The value of the first argument must be between 0 and COLORS. The values that are stored at the addresses pointed to by the last three arguments are between 0 (no component) and 1000 (maximum amount of component).
The pair_content routine allows programmers to find out what colors a given color-pair consists of. It requires three arguments: the color-pair number, and two addresses of shorts for storing the foreground and the background color numbers. The value of the first argument must be between 1 and COLOR_PAIRS-1. The values that are stored at the addresses pointed to by the second and third arguments are between 0 and COLORS.
COLOR_BLACK COLOR_RED COLOR_GREEN COLOR_YELLOW COLOR_BLUE COLOR_MAGENTA COLOR_CYAN COLOR_WHITE
All other routines return the integer ERR upon failure and an OK (SVr4 specifies only "an integer value other than ERR") upon successful completion.
X/Open defines no error conditions. This implementation will return ERR on attempts to use color values outside the range 0 to COLORS-1 (except for the default colors extension), or use color pairs outside the range 0 to COLOR_PAIR-1. Color values used in init_color must be in the range 0 to 1000. An error is returned from all functions if the terminal has not been initialized. An error is returned from secondary functions such as init_pair if start_color was not called.
Note that setting an implicit background color via a color pair affects only character cells that a character write operation explicitly touches. To change the background color used when parts of a window are blanked by erasing or scrolling operations, see bkgd(3NCURSES).
Several caveats apply on 386 and 486 machines with VGA-compatible graphics:
The init_pair routine accepts negative values of foreground and background color to support the use_default_colors extension, but only if that routine has been first invoked.
The assumption that COLOR_BLACK is the default background color for all terminals can be modified using the assume_default_colors extension.
This implementation checks the pointers, e.g., for the values returned by color_content and pair_content, and will treat those as optional parameters when null.