Help with school science fair project...

What happens if you try this on a read carnation? Yellow? Does the type of dye you use change the results? Food coloring, food paste, dye, etc. Equal amounts of dye but different amounts of coloring?

Baconbits has some good ideas. I'd also encourage her to follow those with "Why" questions. What happens if you try this with white, yellow and red carnation? WHY do you think they will be different or same? You can guide her to think of things that might change the results (variables) like the color, size and type of the flowers or the color, type, concentration of the dye or what kind of liquid she puts the dye and flowers in or whatever else she's wondering about. You can then help her focus on the specific questions that she'd want to do this particular project on.

At this age, you can start to introduce the ideas of the scientific method. Simply:
1 ask a question,
2 think/research the question,
3 come up with a hypothesis (what does she think the answer is?),
4 do an experiment to test the hypothesis,
5 analyze the results,
6 conclusion (the results vs. what you thought would happen)

If you want to look at which color will do best, just be sure to clearly define "best". You can also think about time. Does the length of the stem effect how long it takes to get the flower to fully change colors? What if you try a different color on long and short stemmed flowers? What if you use more dye (change concentration of water solution) on long and short stemmed flowers? What if you put salt or sugar in the water? Why? Just 2 of these questions could be a good sized, fun experiment! Don't forget to have a "control" - flowers that you don't add dye (or salt/sugar) to.

What an interesting experiment! I would just pick one or two things to test, so you don't get too many variables.