Analysis of information sources in references of the Wikipedia article "Automotive lighting" in English language version.
To avoid imposing an unreasonable cost to society, NHTSA's lighting regulation continues to allow for the lower cost rear signal (…) Such a configuration can be achieved using just two bulbs and a two color (red and white) lens.
the agency believes improved safety (measured as the reduction in the number of rear-end crashes that resulted in property damage or injury) can be achieved with amber rear turn signal lamps at a cost comparable to red rear turn signal lamp configurations. This is supported by the observation of vehicle manufacturers changing the rear turn signal lamp color for a vehicle model from one year to the next
