Final answer:
A bicycle warning sign is typically yellow, a color used for cautionary signs due to its high visibility and the ease with which it can alert people to potential hazards.
Step-by-step explanation:
The color of a bicycle warning sign is often yellow. Yellow is a color that is highly visible and is therefore used for cautionary purposes to alert drivers and pedestrians to potential hazards, including areas frequented by cyclists. Road safety signs, including bicycle warning signs, are designed to be easy to see and understand quickly by those on the road, hence the use of bright, attention-grabbing colors like yellow.