Actually, a lot of modern cars have amber rear turn signals.

 

Amber rear turn signals have been legal on vehicles manufactured in, or for sale in, The United States since the adoption of the Federal Motor Vehicle Safety Standard 108 (FMVSS 108) on January 1, 1968.

Gov't studies have since proven that amber rear turn signals are recognized earlier by following motorists than red rear turn signsls. But, unlike European countries, there has never been legislation enacted requiring amber rear turn signals in the U.S. or Canada.

 

A few years ago they were pushing to require them to all be amber. Makes way the heck more sense.

 

Ya, there's been talk of this for awhile now, not sure why it isn't adopted. The 06 Sierra has amber and the Silverado doesn't.