Hi, I've done about six DWC grows. Each time the roots looked different. I've come to the conclusion that much of the variation was due to fertilizer and amendment choices. Using any fertilizer with humic or fulvic acid additives tends to discolor the roots. Using "clear" i.e. synthetic liquid or granular based fertilizers tend to keep the roots looking cleaner. Your plants look healthy. If you were having root issues your plants would let you know quickly. I would suggest that you add an enzyme product such as SLF 100 to your water to help breakdown root debris. Also, make sure your water doesn't get too warm. Higher than 80 degrees can be problematic. Finally, if your pump allows for more than 1 aerator stone I would add it. Well oxygenated water is very important.
At this point I would not be too worried about what you're seeing on you roots or your plants. Good luck with your grow!