Let's review the experiment first. The reason the celery sticks get colored is because the celery absorbs the water through its capillaries. This is what plants do in order to get water, which is one of the essential ingredients for growth. The capillaries are located in the xylem of the plant, which is the water transporting tissue in a plant. The capillaries sucked the water so high that even the leaves should have changed color from the colored water. The celery should have sucked all colors of water similarly. If you used red and blue, the blue just may not be as visible in the green celery as the red is. If you peel back a layer of celery for both colors, you should see the veins of the celery dyed that color.

This happened because it has to do with the number of vascular strands - the tubes that move water around inside of the celery stalk (celery is a leaf). If you want more details, please send us more information about your experiment.