Good question, and it's a complicated answer...
On the Federal level, U.S. Forest Service is "Federal Safety Green" but, Bureau of Land Management (BLM) is "Lime Green" which is that bright yellow-green you see. And then National Park Service (NPS) uses plain White.
On the State level, it's whatever the individual state determines and in this case, California Department of Forestry (was CDF now CalFire) uses "Federal Safety Red" as a standard on thier engines and heavy aparatus but that is supposed to change soon, to an overall white with a red reflective stripe running the length of the vehicle.
Individual Counties in California also also have thier own choice and in the case of Shasta County, thier vehicles are red overall.
And for City Fire Departments in this state, it's also thier choice and that's where you'll see a real diversity, Redding has all red, Red Bluff having all white with a blue reflective stripe along thier centerline while the City of Anderson has all "Lime-Green" vehicles, etc.
Regardless of paint color reflective color, all departments nationwide have to follow specific color shades dictated by the Federal NFPA guidelines.