Xi uses traditional Adder black and dark blue with light gray trim.
Per FM:Crusader Clans, page 129.
All Adder ‘Mechs and fighters bear the Clan insignia on the left arm or wing and right leg or empennage. Galaxy insignias are displayed on the right arms or wings and left legs or empennages, while Cluster insignias are worn on the right torsos of ‘Mechs or the fuselages of fighters.
Though the official insignia has the Greek letter Xi superimposed on the Star Adder emblem, a recent and unofficial one shows the Xi superimposed on a golf ball travelling away from a stylized moon.
Per FM:Crusader Clans, pages 118 and 129.