The original space shuttle external fuel tanks were painted white to help protect them from UV rays while sitting on the launch pad for long periods of time. When NASA found out that there was no danger from the UV they stopped using the paint. Here is a quote about the weight savings.
"Subsequent tests found that the paint wasn't vital for shuttle launches and it was abandoned to free up weight - about 600 pounds (272 kilograms) - for additional payload, NASA officials said. "