Too light of green and the salt could be blocking nutrient uptake.
Too dark and they might be getting too much nitrogen in the mix.

If they are the same strain and they look different colors of green then it might be something to look into. I don't use ebb and gro I use Dwc right now but it sounds like the one getting darker green maybe getting more nute concentration. Maybe the water is not circulating back propperly.

Seems the bigger the rez the less you have to worry about the nutes concentrating. As your plants suck up water the rest of the rez gets stronger and stronger. I set my dwc up on a rez to make it easier. It's only 20 gallons for the whole rez change and by the time the plants drink 3 or 4 gallons the PPM's are through the roof. If TinyToon had not mentioned it I would still be trying to figure out why the plants are so dark green and no nute burn on the tip of the leaves.. It effects the flavor in the end too.