Filtering by Color is not working

[monkeybrain]

Every once in a while I get a column that will not filter by color. In my current worksheet, one column will and a different column has that option grayed out. Why is that?

Both columns employ conditional formatting for the colors.

XP, XL2007

 

[monkeybrain]

Also, sometimes the filtered results have rows where cell have no color despite having selected 'Filter by color __'