What is the purpose of these two controls and what are the confounding variables?
You are testing the colored light to see if it impacts the rate of photosynthesis. So you are eliminating no light at all and all the wave lengths in the form of white light. These controls Will give you the parameters or boundaries of how fast photosynthesis will occur. Now you will use only red and see how fast photosynthesis occurs, then you will do the same thing with purple and then green. Now you can compare these values and make sense of the data collected. If you get a rate increase with the red light then you can say red is used more efficiently then the others. If you get a higher rate with purple then you can say it is used more efficiently. Of course if the control (white) light give you the highest rate then you can say white light is the most efficiently used. Only your data will give the answer you are looking for.
