Purple and red stems
This was a common deficiency for my early plants because I started with a strain (Blue Mystic) whose genetics could express purple/red stems and leaves naturally. Unfortunately, purple and red stems are the easiest way to spot a phosphorus deficiency.
My advice is this: the Blue Mysticstrain in particular is wonderful for beginners, so don’t be scared of them. If you see purple/red stems look for other deficiencies and test ppm and pH.