Here is a bit of history from Wikipedia
Until the early 1960s, most front turn signals worldwide emitted white light and most rear turn signals emitted red. The auto industry in the USA voluntarily adopted amber front-turn signals for most vehicles beginning in the 1963 model year,[64][65] though the advent of amber signals was accompanied by legal stumbles in some states[66] and front turn signals were still legally permitted to emit white light until FMVSS 108 took effect for the 1968 model year, whereupon amber became the only permissible front turn-signal colour. Currently, most countries outside the United States and Canada require that all front, side and rear turn signals produce amber light.
In Brazil, Canada, Switzerland and the US, the rear signals may be amber or red. Additionally, red turn signals can also be found in countries with trade agreements with the US such as Mexico and South Korea, as well as in New Zealand.[67] Proponents of red rear turn signals have claimed that they are less costly to manufacture,[68] and automakers use turn signal colour as a styling element to differentiate vehicles of different model years.[69][70] Proponents of amber rear turn signals say they are more easily discernible as turn signals.[71] It has been recognised since the 1960s that amber turn signals are more quickly spotted than red ones.[72][73][74] A 2008 US study by the National Highway Traffic Safety Administration suggests vehicles with amber rear signals rather than red ones are up to 28% less likely to be involved in certain kinds of collisions,[75] a followup 2009 NHTSA study determined there is a significant overall safety benefit to amber rather than red rear turn signals,[76] US studies in the early 1990s demonstrated improvements in the speed and accuracy of following drivers' reactions to stop lamps when the turn signals were amber rather than red,[71][77][78][79][80] and NHTSA determined in 2015 that amber rear turn signals can be provided at comparable cost to red ones.[70]
There is some evidence that turn signals with colourless clear lenses and amber bulbs may be less conspicuous in bright sunlight than those with amber lenses and colourless bulbs.[81]