Another possibility would be to quantify your colors on the RGB scale. This can be easily done for digital photos with a variety of tools. One simple possibility is colorzillahttp://www.colorzilla.com/
Depending on the type of color change observed, there are other possibilities. If you can post your photos here, we might be able to come up with more specific ideas.
Also, I don't see any reason to categorize your project as a "study" that somehow has less challenging criteria than an "experiment". You did complete an experiment, complete with hypothesis, laboratory procedures, results, and conclusions. Numbers are not always the best way to go. If you have photos that tell a certain story, the photos in themselves may be an important result that does not require quantification. Trying to convert a photo into numbers might not inform your audience any better than the photo itself. On the other hand, if certain quantitative interpretation of photos would improve your results section, you can probably get those quantities. Why give people the impression that your project is somehow less rigorous than other projects? Your ideas and enthusiasm are more important than how many numbers appear in your results section. That's my 2 cents.