In the winter time when the daylight is shorter, the lower leaves of the Naja tend to be a bit lighter, slightly olive in color. Your naja was grown in a greenhouse and relies on natural daylight. Increase the time your light is on to 12 hours and the Naja will grow as it does in the summer with brighter green color. The naja in your photo looks good and healthy, believe me I grow plenty of it. If you have a cow pasture nearby add a golf ball size piece of dried manure and I’m sure combined with 12 hours of light your naja will flourish and explode with growth.