Along the lines of the 'ambiguity of gray'? Or that something classified as gray can be said to be inherently undefined? To think about anything, it seems that we have to categorize it in some way. The category we choose unless it is a category of that one item, will be a model also used to describe things or concepts that differ in significant ways from the 'it' we are trying to think about. The fallacy of black and white might then be described as confusing the category with the item itself. The fallacy of gray would be a failure to recognize that gray is a non-category used for 'its' we have not yet been able usefully to categorize as properly belonging with other 'its' on one side of the spectrum or the other.