The definition of "green" is a bit unfair, as it includes things like whether the eggs served in the university restaurants are free range or not (which may be fairer on the chickens but it's not necessarily any environmentally friendlier) and whether or not procurement does an ethical audit (are those clothes made in sweatshops in Asia? — again, not strictly a green issue).
Anyway, I reckon the reason Brunel ranked higher than Essex was because of the colour of their pond at the visitor entrance:
