California title 13 vehicle code suggests that emergency vehicles can have flashing amber to the rear and requires red steady burn to the front. State law mirrors the vehicle code closely. The amber is not a requirement but one of several "allowed practices". The CA Highway Patrol really championed a minimalistic take on lighting with very few lights being left activated in certain circumstances. Sometimes only a single front read steady burn and amber rear flasher were used, other times the rear had a flashing blue light with the amber. Other agencies have done similar and almost all emergency vehicles in CA have rear flashing amber.