Tau Galaxy uses the standard Adder black and dark blue with maroon and gold highlights.
Per FM:Crusader Clans, page 133.
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.
Its insignia is a Greek letter tau superimposed on the Star Adder emblem.
Per FM:Crusader Clans, pages 118 and 133.