Yellow and purple leafe tips

Nice she is showing good bio chemistry within the leaves, senescence is the process of the plant beginning its "death spiral" so to speak. The reallocation of nutrients within a plant's leaves. Some nutrients are mobile some are not. Nitrogen is mobile meaning once nitrogen runs low the plant will start to cannibalize the mobile nitrogen where it is needed elsewhere for more critical bud development. Chloryphyll itself is composed of nitrogen with a magnesium core. This is the yellowing you see. As chloryphyll is removed the other pigmentation remain, this will show through as purples (anthocyanin) or red/orange (cartenoids). It is a sig of a healthy grow done well. Good job. Cold night temps should help if you choose to bring out more purples, but don't push it wait till you are deep into ripening then slam it low see what happens. Good luck.