You have done a really nice job so far of setting up a controlled experiment. I do not know your initial hypothesis, but I presume that it basically says, "if plants are grown in different colors of light, the plants will grown differently." You could have the complete opposite as the hypothesis too. "Plants will grow the same no matter what color light they are exposed to."
A goal of any "good" experiment is to control as many variables as possible. Controls are things that you want to remain the same from trial to trial. You have done a nice job of providing yourself some controls. All of the plants are ferns of the same species. They are all the same age. They are all the same height. All ferns receive the same amount of water at identical watering intervals. The final listed control is that they all receive the same fertilizer.
Other possible controls could be that all plants are grown in the same size pot. They all receive the same amount of soil, and the soil all comes from the same soil batch/bag. All plants should be kept in the same temperature environments, and the humidity levels should be kept identical as well. Because the experimental variable is the color of light, it is important that all of your plants receive the same number of hours of light.
The independent variable is the color of light, and the stated dependent variable is the plant growth; therefore, I would measure growth results by measuring the change in initial height of the plant. Another way to measure growth could be to measure leaf size or number of leaves produced within your time frame.