Color picker title/tooltip is not cleared when field is empty

1. Open any page with color picker.
2. Hover mouse over colored cell.
3. Erase the color from field.
4. Hover mouse over the previously colored cell.
5. Observe the old color still remains, although the cell now displays non-existing color.

Maybe it works as it should so user can remember what it was before, but on the other hand it looks like a bug.
Before in 3.0 it title was just #.
Before in 3.4 there was no title and it appeared as if was transparent.