Food coloring shouldn't affect the way a plant grows, but it should help you see the way water moves through a plant. Carnations are a great tool to see this in action; if you place a white carnation in a glass of water with food coloring it will change the color of the petals. You can even split the stem in half length-wise about 3/4 the way up the stem, place one half of the stem in clear water and one half in water with food coloring and only half the flower should change color.
Paragraph below from: click hereMost plants "drink" water from the ground through their roots. The water travels up the stem of the plant into the leaves and flowers where it makes food. When a flower is cut, it no longer has its roots, but the stem of the flower still "drinks" up the water and provides it to the leaves and flowers.
Okay, now it's time to get technical. There are two things that combine to move water through plants -- transpiration and cohesion. Water evaporating from the leaves, buds, and petals (transpiration) pulls water up the stem of the plant. This works in the same way as sucking on a straw. Water that evaporates from the leaves "pulls" other water behind it up to fill the space left by the evaporating water, but instead of your mouth providing the suction (as with a straw) the movement is due to evaporating water. This can happen because water sticks to itself (called water cohesion) and because the tubes in the plant stem are very small (in a part of the plant called the xylem). This process is called capillary action.
Coloring the water with food coloring does not harm the plant in any way, but it allows you to see the movement of water through the roots to the shoots. Splitting the stem simply proves that the tiny tubes in the stem run all the way from the stem to the petals of the flowers. Our unofficial tests indicated that the blue dye went up the carnations the fastest, followed by the red dye and then the green dye.
Like colored dyes in this experiment, some chemicals that pollute our waters can get into the soil and ground water and contaminate our vegetables and plants growing in the soil. Some chemicals and pollutants, just like the color dyes, may travel up into the plant and affect its health or growth.