EGL does have a shaky reputation for the precision of their grading, at least on pricescpe it does.
This being said, Z is 'fancy light yellow' and yellow it looks. In fact, Z is the last color grade before "fancy yellow". However, this may no mean much since these colored grades have a wider range than the 'white' color grades and the cut of any particular stone makes a lot of difference. Usually, radiant cuts deepen the color, for example.