Convert colours to transparent - PSP XI

Okay, say you have a blank image which is coloured completely with white
and another blank image of the same size coloured completely black. You paint a green dot of with 0% hardness on the same position on both images.

How would you be able to take the information from these images and create a blank TRANSPARENT image of the same size as the previous 2 with a green dot blurring into the transparency (varying alpha channel values) of the 3rd image?

Or to put it another way, how do you filter out things from multiple images leaving things that are unchanged with anything else that differs transparent?

Thanks

Well, you can e.g. select the spot with the magic wand and mask the rest (screenshot). I just can't see the point - why not paint on a separate layer?

http://simplephotoshop.com/px/answers/green-spot.gif

The blurry green dot i mentioned is just an example of something I want to extract from a single layer image filtering out its background. i.e. the background is on the same layer as the desired raster information PSP doesn't perceive it as a background but the human brain can.

I'll give another example of what I want to do for clarity...

Picture an image filled with a 2-colour gradient (lets say red on the left to green on the right). Another image with the same type of gradient but with red on the left and blue on the right. What changes as you compare these images is the colour that red blurs into but what stays the same is the way the red loses integrity as it travels to the right of the image. How do I combine these 2 images giving an image that has red on the left gradually transforming into transparency on the right.

I tried the "difference" blend mode but that gives me black instead of transparency.

Thanks for your response