Do you have this relationship?
Groups::ColorGroupID = Colors::ColorGroupID
If so, then Count ( Colors::ColorGroupID ) could be defined in Groups to count the number or related records in each group.
That's how I set it up and was scratching my head about why it wasn't working because it should have been pretty simple!
But I have since worked out what "Evaluate this calculation from the context of:" means! .... I had multiple occurences of the table in question
You learn something everyday :)