Business is Business
Believing that the foe can be intimidated by the sight of a solid wall of assault 'Mechs, the Thumpers have adopted a highly visible color scheme of yellow with black trim.
Per Mercenaries Supplemental, page 63.
Zeus, Mercenaries Supplemental (color plates)
The Thumpers' insignia is a white fist on a purple background; originally a right hand on a blue field, at some point the insignia made a mysterious change to a left fist on purple. For acts of selfless heroism that save Thumper lives, individuals are allowed to display the logo with the fist pointing downward. Per FM: Mercenaries Supplemental, page 63.