The Storm Giants are illustrated as wearing bone-colored armor (including trim), but which would probably be more acurately described from photographs as very pale green (like the old GW paint "Rotting Flesh" ). The chapter symbol is a stylised red thunderbolt, pointing upwards:
And while we're on the subject...
The Storm Lords wear armor that is white on the left side and red on the right, with a yellow helmet (although helmet may indicate company color per Codex Astartes). The chapter's symbol is a skull superimposed over three red lightning bolts:
The Rampagers wear red armor. The chapter symbol is of a fist clenching a yellow lightning bolt:
The Marauders wear yellow armor. The chapter symbol is a flaming sword:
The Destroyers wear "acid yellow" armor with a red chevron pattern down the arms. Their symbol is two crossed lightning bolts. (Sorry, no pic).