80% of the students felt a calming effect when being exposed to the colour blue irrespective of gender. I can imagine that 20% reported not feeling calm when exposed to the colour (it doesn’t really say).
To repeat this test and improve its validity. You need do this in a randomized crossover fashion. You need to randomize the students into 2 groups.
One group gets exposed to the Color blue and then gets exposed to another color. The other group gets exposed to another color first and then the color blue. After some period of time (a day). You expose the group that saw blue first to the different color and the group that saw the different color, first to blue. So you switch the order of exposure. By doing so, you are comparing the color blue to another color that may stimulate similar or different effects. This way you truly determine if blue is calming.
You also need to ensure that the students behaviours, activity, and diet are controlled. If someone sleeps longer, eats a certain food, is less agitated to begin with might feel different about seeing a color. For instance if I’m upset and see a cool color such as blue, it may calm me. If I’m not upset and already calm and happy, seeing the color might do nothing for me. This is a really difficult thing to assess as it subjective to the individual. Perhaps it’s best to ask them the question after you ensure they have eaten, rested (slept) for the same amount of time, and are not particular in an anxious or agitated mood. But good luck controlling for all that.