Why are the two strings of Guqin different in color?

It has been cultivated for a long time. The inner core of the first and second strings is different from other strings, and the materials of different brands are different. It has been cultivated for a long time. Generally speaking, the green core is copper. After the strings are oxidized, the sound will be more mellow and the smell of steel wire will be reduced. Guqin, also known as lyre, Qin Yu and lyre, is a traditional plucked instrument in China, which has a history of more than 3,000 years and belongs to eight-tone silk. Guqin has a wide range, deep timbre and lingering sound.