Howdy nzic, I'm a DWC grower as well (on 6th grow). The discoloration does not look too bad. How old are the plants and how far away is the light, light bleaching can happen to young plants. I get small areas of discoloration, ranging from a yellowish to a bluish shade. Sometimes, the veins in the leaves will be slightly lighter than the rest of the leaf. One thing that You might check on is the temperature of the water. This can make a difference in the plants up take of nutrients as well as pH variations can. I use ice bottles to keep my reservoirs between 63° and 70° F, the bottles (.7 L size) keep the temperature down for two to three hours during the 'lights on' time.
Your plants look very good and healthy over all, good luck in Your grow.