Most of the time, the Stormtroopers don’t actually die, they are simply incapacitated. When Stormtroopers are hit by blaster shots, they are flung backwards due to the kinetic force of the shots. The Stormtroopers die very easily because the plot demands that our intrepid heroes always succeed against the enemy. Additionally on the Stormtrooper Wookieepedia site, the White armor signified unit solidarity and became a distinguishing signature characteristic.
However, white was the primary color as listed at Wookieepedia. We do see in episodes I, II and III that other colors were used to better blend with other environments. Thus their armor is most effective at dispersing heat as thats how Blasters do damage.
Its lighter armor and most kills on Stormtroopers are from hits to the bodysuit not the plate arnor.