Final answer:
Orange is the best color to mark a shelter for rescuers due to its high visibility and common use in safety contexts, making it stand out in various lighting conditions.
Step-by-step explanation:
In the context of emergency situations where visibility is crucial for being located by rescuers, certain colors are more effective than others. Among the options provided, orange is the best choice for marking a shelter. It is not only a high-visibility color but also is frequently used for safety purposes such as on caution signs and the uniforms of emergency workers. This makes it stand out in various light conditions, thereby aiding rescuers in finding the shelter more easily compared to the other color options. While red and yellow are primary colors and very vibrant, they may not stand out as well against certain backgrounds. On the other hand, blue tends to blend in with natural surroundings like water or the sky, making it less ideal for this purpose.