The Black Watch usually paints its 'Mechs green, with bands of the Black Watch Tartan randomly encircling the 'Mech. Additionally, all 'Mechs have the Black Watch insignia molded into the armor of the right arm shoulder assembly.
Per FM:Comstar, page 100.
Pillager, CB: Miniatures Rules (p 23)
Highlander, FM: Comstar (plate 6)
Black Watch, FM: Comstar (front cover)
All 'Mechs have the Black Watch insignia molded into the shoulder of the right arm assembly. Per FM: ComStar, page 100.