You might be having a phosphorus deficiency is usually accompanied by bright red stems (though not always), though if you have red stems but no other symptoms, it’s typically not something to worry about
leaves darken (turning a dark green, blue or grayish color) and may appear shiny
leaves may start turning yellow in places if the phosphorus deficiency is left untreated, or if the deficiency is combined with other nutrients deficiencies and/or pH problems. However, yellow leaves is typically not associated with the beginning of a phosphorus deficiency.