How was plant growth measured in the experiment and what were the plant
data for the three treatments? Although often used by students, height
is often not the best measure of plant growth. Plants that grow tall
may be thinner and gain less weight than shorter plants. A better
measure of plant growth is shoot fresh weight or dry weight.
Covering a plant with colored plastic will significantly reduce the
amount of Photosynthetically Active Radiation (PAR) that a plant
receives. The rate of photosynthesis and plant growth is often directly
proportional to the amount of PAR the plant receives. PAR counts the
number of photons between 400 and 700 nanometers in wavelength, which
is visible light. In photosynthesis, the energy of a photon is not
important; basically a red photon is just as good as a blue photon.
Therefore, one would expect that a plant receiving full sunlight would
grow better than one receiving only the red or blue wavelengths in
sunlight. To fairly compare plant growth under different colors of
light would require using a PAR meter so that all plants received the
same amount of PAR. Unfortunately, PAR meters are expensive so are not
usually available for student projects.
I believe Dick Chambers has possibly the best explanation - that the
blue plastic covering the cup provided a better environment for the
plants than the uncovered plants that received full sunlight. The
plastic covering could have increased the temperature or retained more
water. The appropriate control treatment would have been a cup covered
with clear plastic.
Another possibility is that the results occurred just due to chance.
There should always be replication of three or more pots per treatment
in order to take into account the natural variability in plants and the
possibility of a plant dying or growing poorly in a treatment for
reasons unrelated to the experimental treatment.
Questions about projects like yours come up frequently at madsci.org -
http://madsci.org/circumnav/index.html
The MadSci.org botany archives can be searched for light color.
David R. Hershey