3000-4000k. Preferably 3500k. This will work for veg and bloom. Forget spectrums besides this (although CRI should be 80 or 90 CRI), and focus on achieving as close to 1100-1200 PPFD per square meter as you possibly can. After 1200, plant growth slows and at 1500, photoinhibition can take place. I achieve about 1000-1100 PPFD per square meter using only 241w of COB.