coloured maskblob

I would like to have your advice for a basic question; how to generate 'maskblob' on colour background.

On PsychDemo, for example,
- AlphaImageDemo.m
- GazeContingentDemo.m
use alpha channel to create the maskblob but with gray background colour (gray = GrayIndex(screenNumber);)
Is there any way to chnage the colour of the maskblob along with any background colour?

What I would like to do is to generate a gaussian mask (or a window) to see stimulus without any edge effect,
say, images of any objects or faces on coloured background with blurred image frame.
What would be the best method?

I thought of using blending two colour images by
Screen('BlendFunction', w, GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
Screen('DrawTexture', w, img1);
Screen('DrawTexture', w, img2,[],[],[],[],[0-1]);
But I am uncertain.

I would be grateful any of your advice.
Many thanks.
Ka