Purple stems can mean a few things.
Everyone wants to jump on magnesium deficiency but that's usually not the case. If you have a plant with purple stems and it looks perfectly healthy, it's fine. Keep in mind the stems don't process photosynthesis therefore they don't need to be entirely green to do their work. Seedlings almost always have purple stems.

As far as phenotypes, turtle couldn't be more correct.

Think of the purple stems as me having brown eyes, it's just a physical trait.