Symptoms
Leaves with P deficiency are smaller than normal and show purple tints on their dark green leaves, especially on the middle of the leaf blade.
K deficient leaves are even more stunted and their tips are scorched.
Reasons
P and K deficiency vs. optimum supply
Spring cereals may show reddening/purpling of older leaves in cold weather. Root and stem damage or poor soil consolidation can cause similar symptoms, too.