Just had a look at 'Severity of Flight Delays' in the gallery (http://www.tableausoftware.com/learn/gallery/severity-flight-delays). If you change the Arrival Delay filter (say reduce the top limit to 25), then the color mapping (i.e green to red) changes, which is most confusing. What was green can suddenly becomes red! I assume there is a way to fix the color scheme using the min & max limits?
Yep, under the advanced section of the color dialog