Maybe take a look at Apple's code:Explained Adventure. At section "Loading Shared Character Assets", they use:
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
sSharedIdleAnimationFrames = APALoadFramesFromAtlas(@"Boss_Idle", @"boss_idle", kBossIdleFrames);
// (Load other animation frames)
sSharedDamageEmitter = [SKEmitterNode apa_emitterNodeWithEmitterNamed:@"BossDamage"];
sSharedDamageAction = [SKAction sequence:@[
[SKAction colorizeWithColor:[SKColor whiteColor] colorBlendFactor:1.0 duration:0.0],
[SKAction waitForDuration:0.5],
[SKAction colorizeWithColorBlendFactor:0.0 duration:0.1]]];
}