At a four way stop
yes
To inform the public.
The signs usually are yellow with black outline or trim for example sharp turn. The background would be yellow but the arrow would be black If you are in Europe, such signs are triangles with red borders and a white or yellow background. Scandinavian countries use the yellow, while most of Europe uses the yellow for construction.
On the wall
That depends on what color the curb is. White is for passenger loading or unloading only. Yellow is for cargo loading. Red means absolutely no parking. Green is usually for a limited time. Blue is for handicapped parking only. Watch the posted signs when you park.
Regulatory purposes.
Warning signs are usually a diamond shape. The color of the sign is yellow in order to caution drivers about an intersection ahead, bicyclists or wildlife that may be near the road.
yellow
It depends on the laws of your state
If the reduce speed signs are posted, yes.
The Open sign is a way of indicating to the general public that a store or business is now accepting customers. These signs are usually in the same area that the business hours will be posted.
If you try you will be shot. There are signs posted around the entire area, and it's patroled by aircraft, usually helicopters with heavily armed military personel.