al_premul_rgba - Allegro 5 API

       This is a shortcut for al_map_rgba(3alleg5)(r * a / 255, g * a / 255, b * a / 255, a).

       By  default  Allegro  uses  pre-multiplied  alpha for transparent blending of bitmaps and primitives (see
       al_load_bitmap_flags(3alleg5) for a discussion of that feature).  This means that if you want to  tint  a
       bitmap  or  primitive to be transparent you need to multiply the color components by the alpha components
       when you pass them to this function.  For example:

              int r = 255;
              int g = 0;
              int b = 0;
              int a = 127;
              ALLEGRO_COLOR c = al_premul_rgba(r, g, b, a);
              /* Draw the bitmap tinted red and half-transparent. */
              al_draw_tinted_bitmap(bmp, c, 0, 0, 0);

       This function can be called before Allegro is initialized.

       al_premul_rgba - Allegro 5 API

al_map_rgba(3alleg5), al_premul_rgba_f(3alleg5)

Allegro reference manual                                                                 al_premul_rgba(3alleg5)

       5.1.12

              #include <allegro5/allegro.h>

              ALLEGRO_COLOR al_premul_rgba(
                 unsigned char r, unsigned char g, unsigned char b, unsigned char a)