>> Maybe better to do what Drew proposed: to distinguish the odd groups
>> from the even groups, i.e. to have only 2 additional faces
>> (a brighter face like the current isearch-group-1 for the odd groups,
>> and a darker face for the even groups).
>
> I like the idea.

Implemented now on the trunk where isearch-group-odd is created
from the isearch-group-1 face, and isearch-group-even
from the isearch-group-4 face.

The result is quite nice: