OpenGL glBlendFuncSeparate - objective-c

Question

I need some help with OpenGL textures masking. I have it working but need to find some other blending function parameters to work in other way.
Now I have:
//Background
...code...
glBlendFunc(GL_ONE, GL_ZERO);
...code
//Mask
...code...
glBlendFuncSeparate(GL_ZERO, GL_ONE, GL_DST_COLOR, GL_ZERO);
...code...
//Foreground
...code
glBlendFunc(GL_DST_ALPHA, GL_ONE_MINUS_DST_ALPHA);
...code
Now it sets foreground's opacity to 0 (fills with background texture) where mask is transparent. I need it to react to mask's colors. I mean something like setting foregrounds opacity depending on mask's color. For example if mask is black (0.0,0.0,0.0) then the opacity of that place in foreground is 0 (is filled with background), and if mask is white (1.0,1.0,1.0) then the opacity of foreground is 1 (not filled with background). It can be in reverse consequence (white = opacity 0, black = opacity 1). I just need it to work depending on color.
My current result's visualization bellow.
Background:
Mask (circle is transparent):
Foreground:
Result:
And I want it to work like this:
Background:
Mask (circle is white, background is black):
Foreground:
Result:
So that later it could be used like this:
Background:
Mask (circle is white, background is black):
Foreground:
Result:
Attempt with #Gigi solution:

Answer 1

Perhaps this is what you want:
1) Clear the destination image:
glClearColor(0.0, 0.0, 0.0, 0.0);
glClear(GL_COLOR_BUFFER_BIT);
2) Draw the background, masking out the alpha channel:
glColorMask(GL_TRUE, GL_TRUE, GL_TRUE, GL_FALSE);
3) Draw the "masking overlay", masking out the color channels:
glColorMask(GL_FALSE, GL_FALSE, GL_FALSE, GL_TRUE);
4) Draw the foreground, enabling blending:
glEnable(GL_BLEND);
glBlendEquationSeparate(GL_FUNC_ADD, GL_FUNC_ADD);
glBlendFuncSeparate(GL_ONE_MINUS_SRC_ALPHA, GL_SRC_ALPHA, GL_ONE, GL_ZERO);
Note: The overlay image must have the alpha channel specified.