Charles Spurgeon Commentary Numbers 4:12-13

"and they shall take all the vessels of ministry, wherewith they minister in the sanctuary, and put them in a cloth of blue, and cover them with a covering of sealskin, and shall put them on the frame. And they shall take away the ashes from the altar, and spread a purple cloth thereon:" — Numbers 4:12-13 (ASV)

A royal altar is this, always grand and glorious in our eyes, covered with a purple cloth.