What are the rules for the color of the character seal? Blue seal has any special instructions?

The sealing clay used for character and painting seals is generally made of vermillion (slightly purplish red) sealing clay or vermillion fat (slightly orange) sealing clay, in addition to antique sealing clay (purple sand color, dark ochre), and some special purposes (mostly used in religion) of gold and silver sealing clay. The blue sealing clay is generally used for inscribing private letters with other people during the period of the Tingxu guardianship, and the black sealing clay is used when there is a funeral in the family.