It's weird, because as soon as i switched to FF soil and fluoros, all my stems turned red. Red stems are also a sign of overfert, so I'm thinking that the rich FF soil is causing it. They're not fert burned, but are defininetly getting the max amount prior to burning. Excellent soil, engineered to give max nutes possible without burn, and will provide nutes for the first couple weeks of veg without worring about any deficiencies.
We need more info to diagnose correctly. Age? Size? Fert?(if any) Soil? Hydro? Temp?