I use png images so I naturally have alpha channel in my graphics layers for parallax scrolling. Whenever I change the depth of my unit that can be either behind or in front of bushes layer the edges of the bush change their color value a bit. It isn't ugly or anything but I can notice a change just because the queue of layers changes.

Heh I guess I found the bug..

That circle is drawn using
al_set_blender(ALLEGRO_ADD, ALLEGRO_ALPHA, ALLEGRO_INVERSE_ALPHA);

but scaled sprite images use this:
al_set_blender(ALLEGRO_ADD, ALLEGRO_ONE, ALLEGRO_INVERSE_ALPHA);
The bug was with drawing scaled image that didn't set blender at all, so at some points it used the first one and at some points the second one.

That blender function is quite confusing. I had to use the first one mentioned for circle drawing to have custom transparency but images that already have alpha channel had to use some other blender set.