Why are rescue blankets silver? Aren't they supposed to warm up the person inside

http://i.ebayimg.com/t/Emergency-Rescue-…

They DO keep you warm, especially after being submerged in frigid temperatures, etc...
Their color comes from the material, which is mylar (the same thing they use in balloons) and they use this material because it retains heat.

They are also used after marathons to regulate a runners body temperature, which changes drastically after the course is over.

ETA: ERBs are also used to keep heat out, as well as in.