Here is one of the ideas I mentioned that we were going to do as part of our Food Theme of the Week. It is from superteacherworksheets.com. My husband laughed at me a little as we did this science experiment because I was just as interested as my kids at seeing if it worked because I never did it in school when I was little.
8 hours later, here were the results. The blue and green had the most noticeable changes. The yellow was hard to see and the red didn't show up as strongly (especially in this picture).
My kids thought it was pretty cool though. Maybe someday we will try the same experiment with a white carnation, which is sure to show all the colors.