dark green/blue veins?

Last night I plugged everything into a timer so that they'll now go from 24/0 to 18/6. They are in their third week of vegging from the first sign of a sprout. I just went down stairs to check on them and give them some water and although the leaves are very lush and green, the veins are now a prominent dark green/blue. It was never that pronounced before.

Any ideas? They're in fox farm ocean soil and no nutes just watering when they're dry.

Fox Farms Oceans Best is known for being a little too "hot" for most young plants. You may just want to try a flush and see if that helps.