Sigma paints its equipment with yellow and red accents over the standard Adder black and dark blue.
Per FM:Crusader Clans, page 132.
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.
A Greek letter sigma superimposed on the Star Adder emblem is the Galaxy’s insignia.
Per FM:Crusader Clans, pages 118 and 132.