Science Buddies: "Ask an Expert"

My science fair project is about the effect of color on a human's time perception. They will be observing a website with a blue loading screen, and will be asked to state the efficiency of the website on a scale of 1-10. The control will have the same process except with a grey loading screen. My question is could the human participants I have participate in both the experimental and control groups? I do not have a very big sample size so I am worried about the reliability of my project. Would my project be invalid if the participants observed both websites?

You run the risk of 'order effect', that is your participants will learn something from the first page they try. You want half the group to try the blue first then the gray, and the other half try gray first. Select them randomly into the two groups and see what happens. As always, more participants is better than fewer.

Howard