Completely normal. Towards the end of the flowering cycle, nitrogen levels drop causing the leaves to yellow/redden/purple. This is the occurs naturally and it would stand to reason that the plants that are yellowing are "early risers", finishing at around 60 days. Pay attention to the brown hairs and trichomes on the plants with the yellowing leaves.
All I see here is healthy plants.
__________________
Si Vis Pacem, Para Bellum- "If You Seek Peace, Prepare For War."
"Those who would sacrifice liberty for safety deserve neither." ~ Benjamin Franklin
ΜΟΛΩΝ ΛΑΒΕ