Which color is not used on day boards according to the chart mentioned?

Day boards, used as navigational aids on water, are specifically designed and colored to convey important information to mariners. The colors used on day boards typically include red and green, which indicate safe passage and hazards, respectively. Yellow is sometimes employed for marking certain types of navigational aids, such as mooring buoys or other significant features.

Blue, however, is not a standard color used in day boards. This distinguishes it from the others, which are established colors within the maritime signaling system. While blue might be associated with specific navigational markers or signals in different contexts, it does not appear on the day boards as indicated in the navigation charts.

Thus, identifying yellow, red, and green as the key operational colors used in day boards helps clarify why blue is not included in this category. Understanding these color meanings is essential for safe navigation and compliance with maritime regulations.