There is a difficult problem that I am dealing with now.
In the system that works with data there are filters. One of them is displayed as a list with the "Select all" / "Deselect all" buttons and the user can filter some adjustments that he needs (as the example would explain in colors): you can select options such as red, blue, green, etc. and a special option that means "without color" – it is not equal to "transparent", but more to "we do not have data about the color of this object" or "this object should not have a color".
So the question is: How can this option be marked in the list along with other options, but can not be "lost" between them? Basically, how can it be named? "No data" is not clear, "no color" is not described correctly, we just do not know if there should be one …
Putting this option aside or creating another "level" of filtering, defining whether the object should have a color or not, is not a solution. The user must be able to select at the same time, p. "without color" and "red".