Thanks for the suggestions!
But I don't want to use Allegro 5.1 (concerning unstable), so the shader addon is no solution. And the direct use of OpenGL is no alternative for me so far.
My quick solution is to draw the bitmap and then draw over a primitive (in this case a circle) with alpha channel. In the example image you can see the result (right), but in fact I wanted to color the full bitmap (with the red crown).