Stem colour differences.

I'm using up my auto seeds and germed all 5 of my BSB Sweet Tooth Autos.
All are identical with the exception of one has a bright red/purple stem. Other 4 are pure green.

Is this just pheno differences? They all had the same start in life, just seems a bit odd.

Just a pheno difference, probably. Like I went bald in my forties, but my dad and two brothers didn't