That can just be genetics. I run a RDWC, currently with 4 plants. 3 are almost done, 1 is ready for harvest. The one I'm harvesting has green leafs still. The other 3, that have about a week to go, 2 are yellowing, 1 is turning black.
If you want more purples and crimson you need to lower the temps as well. It's just a visual aspect does not affect the buds.
Keep up with just water and the rest of the plant will start to fade. They might have had a lot of energy stores in them.
Best Of Luck!