Numbers 4:13
And they shall take away the ashes from the altar, and spread a purple cloth thereon:
Cross-reference
In Numbers 4:6-9, the ark, table, and lampstand are covered with blue cloths — similar procedure as covering the altar here, but with different colors.
In Numbers 4:11, the golden altar is covered with blue cloth — the same covering method but a different color than the purple used here.
In Numbers 4:12, utensils are covered with blue cloth and badgers' skins — here the altar is covered with purple, showing parallel covering procedures.
Exodus 27:3-5 describes the bronze altar's utensils and grating — the same altar that is covered here during transport.
Exodus 27:7 describes staves for carrying the altar — the same altar is covered here before transport, implying the use of those staves.
Leviticus 6:12 commands the fire to burn continually on the altar — the same altar from which ashes are removed and which is covered here.
Leviticus 6:13 restates that the fire on the altar must never go out — the altar covered here is the one with the perpetual fire.
Exodus 39:1 lists purple among the colors for service cloths — the same purple cloth used to cover the altar here.