What color is the cover of Sikuquanshu and what is its significance?

In order to be beautiful and easy to identify, Sikuquanshu is decorated with color separation, with green longitude, red history, white (or light blue) division and gray-black collection. The determination of four colors depends on four seasons: spring, summer, autumn and winter. Because the catalogue of Sikuquanshu is the outline of the whole book, the yellow color representing the center is adopted.