Remove background and merge images matlab

I am trying to do what the title says. I have a very long code that I run several times to obtain different pictures like the 2 here:

After I run the code I save the picture (although I don't have too, but i think its the easiest way). Now what I want to do is remove the black background(make it transparent) and superpose the images, the blue and yellow stuff to see if there is some overlap

what I did is save, those as A and B and use this:

A=imread('A.tif');
B=imread('B.tif');
C=imfuse(A,B);
imshow(C)

And I get this picture:

Of course the black background is still there (cause I don't know how to remove it). By making the black background transparent I will be able to get green when it overlaps. Is there a way to do that with the processing toolbox?

Hope someone will be able to help.

Thanks guys