What is the typical shape and color of warning signs?

Warning signs are intended to alert drivers to potential hazards or changes in driving conditions ahead. The typical design for warning signs is a diamond shape and they are predominantly colored yellow. This color and shape combination is easily recognizable and is standardized to convey caution. The diamond shape helps to capture attention and often indicates that drivers should be prepared for a decision or action, such as reducing speed or being alert for potential hazards like curves, intersections, or pedestrian crossings.

In contrast, other options feature different shapes and colors that do not align with the established conventions for warning signs. For example, round signs are usually designated for railroad crossings, and red is primarily used for stop signs and regulatory notices, while rectangles and squares in blue or green serve different functions such as information or guide signs. Thus, the diamond shape and yellow color uniquely define warning signs, making the selected answer accurate and appropriate in this context.