Normal. Consider them fall colors. Could be genetics as well, but usually as to do with chlorophyll. Cooler temperatures inhibit chlorophyll production. Chlorophyll, you might remember from 6th-grade science, is the plant component vital to photosynthesis. Different genes and different cannabinoid ratios, flavor profiles, and anthocyanin are all factors. The most prominent variation to green cannabis is purple as seen in your pic. Think of the tree leaves in fall. As temperatures drop, they change from green to red, orange, yellow, or gold. Cannabis doesn’t produce the colors until the latter half of the flowering stage, with a few exceptions.
If You really want to see your plants change colors wait until the last two weeks of flower. Drop the temps during lights off to 58-60 and lights on period move the temps back to 70-75 (keeping it cool during lights on) Tricking the plant to thinking the end is coming. She will start showing fall colors and start ripening