Change the definition of a color.
color_number
The number of the color to be changed (0 - (COLORS-1)).
r
Red component (0 - 1000).
g
Green component (0 - 1000).
b
Blue component (0 - 1000).
When init_color() is used, all occurrences of that color on the screen
immediately change to the new definition. This function is a no-op on
most terminals; it is active only if can_change_color() returns true.