I guess the real question is why would you WANT to use white to the rear?
White to the rear became popular a number of years ago when corner strobes (not LEDs) were starting to show up. The coloured strobes sucked in terms of colour (red looked pink, etc.). Also, someone got it in their head that you could see white farther away than any other colour, which with strobes was probably correct.
LEDs have changed that all as white is not really the best colour any longer. Red tends to be the brightest colour in terms of intensity.
Some reasons NOT to use white to the rear;
- it violates SAE J2498 which is an industry-standard for emergency warning
- it violates NFPA (a fire standard, but still worth considering)
- white provides no immediate recognition and can be confused for other lights
By "no immediate recognition" I mean the often-forgotten 2nd stage of warning. The first stage is detection - a driver has to physically see your lights. The next is recognition - the driver has to understand what the light means. The 3rd stage is response -the driver has to take some action (brake, swerve, etc).
Even in the strobe days, white provided good detection, but very poor recognition. If you ask someone what a flashing white light means, they won't be able to give you a good, instant answer. If you ask someone what flashing amber means, everyone knows it means a hazard ahead. If people can see your lights but can't understand the message they're sending, then the lights are just about useless. It would be like making stop lights blue instead of red. No one would know what they mean, even though they can physically see it, so they're not likely to stop.
I would suggest amber to the rear. It is a great universally-recognized "hazard" colour, stands out from a sea of brake lights (as opposed to red which can blend in), and provides better all-weather light penetration than blue. Have some red and blue to the rear, sure, but have some ambers as well.
Some officers I've dealt with say "I don't want amber because I want drivers to recognize me as a police car". Ok, I get that, but if they can see amber farther away than red or blue, then who cares if they initially think you are a police car, fire truck, or garbage truck. The key is to get the driver slowing down so they don't hit you. As they get closer, they'll see the reds and blues and recognize who you are.
You might be interested in this study (PDF);
https://copy.com/9zbhr302uY4qyxeR . Note that it's from a few years ago so deals mostly with strobe and halogen, but the basic principals are the same.