They look just a tad hungry. Not a bad thing unless they start getting a lot more yellow, you could probably up the feed to 700ppm and get some of the color back but you don't want them to get too dark green either.
Its probably just the color balancing on your phone camera.
Not to much of an issue thats anthocyanins. Once you lower the nitrogen there is less photosynthesis in some leaves. Add to that some genetics produce more than others and if you lower temps to around 70 can be even nore pronounced. I do this last week or so of my grows to being out the colors. Anthocyanin is not present where chlorophyll is. Some strains produce it more in the buds and other the leaves. These will be purple, blue, red etc. Not an issue to see these colors but you may want to keep an eye on your nitrogen if they start yo yellow to much.
Well that's them been fed again still looking good. all got fed between 600 and (800ppm for the biggest one) they are about 3 weeks 2 days into flower now.
