«فَأَبْرَزَ مِنْهُ رِقَاعًا قَدْ كُتِبْنَ بِأَلْوَانٍ»
He drew out slips that had been written in different colours.
Riqāʿan is an indefinite accusative direct object. The following verbal clause qad kutibna bi-alwānin describes it. Kutibna is passive; the feminine plural suffix refers back to the slips and functions as the passive subject.
With the perfect passive, qad underscores an accomplished state: the slips had already been written. The narrative draws attention to the prepared documents rather than naming the scribe.