I am inclined to agree with Pete, what type of artificial light and with the hours stated that they are on, along with a few hours of sun, it could be a bit too much light.
But you really won’t kill a plant with too much light, it is just going to impart a brighter green or yellow green color to the leaves.
Are you using a well balanced fertilizer providing enough nitrogen. A little more nitrogen will increase the green. I use a 20-20-20 blend with a bit more calcium and magnesium then most blends. My lights are on though for just 10 hours a day now. I gradually give them more light until May 1st. They get 14 hours a day in April. Then they go outside for the summer.
I fertilize every other week at 1/2 tsp. Per gallon.