Laws vary but amber should be fine for front or rear. At night, you would need a red light either instead of, or in addition to an amber rear, but I've seen very little reason why an amber can't be used. Obviously, many vehicles use red for hazard lights as they don't have amber on the rear. Using amber has the benefit of not only being brighter, for the same wattage, but not being confused/blending in with brake lights more than the emergency vehicle aspect.

What is illegal in every place I've researched, and for sound reason of being reserved for emergency vehicles, is alternating flashing red (or white on front) lights. In many places, maybe all of the US, front flashing white lights are technically allowed as vehicle turn signals (which double as front 4-way flashers), again as long as they aren't alternating. Alternating flashing amber lights are pretty easy to get away with in any location/position.