I remember the time I get involve in the krita project, my first visible feature was the invert filter, but it took me three months to write it. Mostly because there was a lot of infrastucture work to do before 
But the point is that today’s filters are simple because I wrote them to test that writting filters outside krita was possible.
I will still use the original image of the Villa Savoye of Le Corbusier, mostly because I realy like the work of this guy:
Here is the result of a filter that will filter to leave only the minimal channel:
Here is the result of a filter that will filter to leave only the maximal channel:
This filter remove a color from the image and transform it to alpha to make it transparent, here I have selected a tone of blue and I have nearly remove the sky with a next gradient:
re; the first 2 images;you say you leave only one channel. This means to remove the other 2. Then why does the end result still have all the primary colors there?
I am not sure what you mean, but what the max/min filter do is find the minimum/maximal value for each pixels, and keep it, that’s why we still have all the three colors. And they are some part of the house which are yellow corresponding to pixels where red and green are equal. (note that the herb is really dominated by red and that it is not a bug)