The problem with this technique of very small "sensor" dots is antialiasing. Since antialiasing affects the edges of a shape it may affect the whole shape if that becomes smaller. Better to use nested sprites and collision detection for small sensors.
Another - much more precise - technique if you want to check for color collision is to use the hue at _ reporter. you can check the color at a specific single point. That way you can find colors that escape color collision detection.
I've hit an issue with using hidden nested sprite - it doesn't stay hidden if I try and use it as a clone and its parent is showing.
I can't think of a way of getting a reference to the nested sprite and therefore can't tell it to hide or ask it if its touching another sprite
[edit - I've discovered my parts reporter so trying that out]
I can get a reference to the clones nested sprite and then tell it hide (and use reference to ask it if its touching other sprites later