al_set_blend_color - Allegro 5 API

       Sets  the  color  to  use  for blending when using the ALLEGRO_CONST_COLOR or ALLEGRO_INVERSE_CONST_COLOR
       blend functions.  See al_set_blender(3alleg5) for more information.

       al_set_blend_color - Allegro 5 API

al_set_blender(3alleg5), al_get_blend_color(3alleg5)

       5.1.12

Allegro reference manual                                                             al_set_blend_color(3alleg5)

              #include <allegro5/allegro.h>

              void al_set_blend_color(ALLEGRO_COLOR color)