As a Williams alumnus: the major color groups correspond to the "divisions" of courses at Williams -- the administration groups all departments into three divis

As a Williams alumnus: the major color groups correspond to the "divisions" of courses at Williams -- the administration groups all departments into three divisions, and requires that students take a certain number of classes from each division. So the chart faithfully reproduces the decision of the Williams College administration to include Psychology in Division II (Social Sciences) rather than Division III (math and hard sciences).