Blue because 1930s United States was going through the Great Depression and a lot of people were going through poverty and had a lower quality of life which makes people sad. Also when you think of depression, you think of blue because depression makes you sad and sad is associated with blue.
B. Individual productivity is necessary for group productivity.
Imagine a work in a group, where the people all receive their personal tasks that they will later incorporate into the group work. If each person is not productive on their own task then the whole group will not be productive!