Amber has been scientifically proven to be easier to notice, and mildly safer than using the same bulb as the brake light to indicate a turn.

Here’s a paper on it. https://crashstats.nhtsa.dot.gov/Api/Public/ViewPublication/811115

This paper contains data relating to the effectiveness of amber turn signals by comparing striking and struck cars with the same configuration (amber or red), and the odds of not getting struck with an amber turn signal equipped vehicle is always about 4-8% better than otherwise.