Visual cues only work with expression that return numericals. Your second expression is returning a string.
But you can use a background color attribute expression (click on the small plus sign next to your expression in expression tab) and enter as defiition something like
=if(avg(field1) < 0, lightred() )
i.e. you must create an expression that returns a color (can also be created using rgb() or argb() functions).
Hope this helps,