You can use it to make multi coloured cut flowers, put the food colouring in the flower water and it is drawn up into the flower changing the colour of the petals. I dont really like cut flowers that much so I made some blue celery last year.

great stuff for kids:
Make colored playdoh for kids.

colored cornstarch and water, its a lot of fun, it turns liquid in your hand, but solid on table tops, kids love it.

add it to water in a spray bottle and do art. really cool to make flowers from coffee filters.

add to bubbles to make bubble art.

If i remember correctly its possible to make home-made play doh, that requires food colouring.

You can drip a few drops in the tank of your toilet… If the colour bleeds into the bowl you know you’ve got a leak.

You could use it to color water that in a spray bottle to spray on snow sculpture. Fun for kids.

You could use it to color water in a spray bottle to spray on snow sculpture. Fun for kids.

Kids can use it when making salt (or similar) crystals rather than just making “clear” ones.

Make face paint with cold cream, cornstarch, some water, and food coloring to color! It also makes good finger paint with just cornstarch and water. You can also use it to color other foods: pasta, pudding, rice, etc.

Take it to your local school, it can be used in experiments and arts.

Vandalize local fountains.

Okay, that’s a terrible idea. But food coloring is an effective way to locate leaks, as someone has noted.

A friend of mine collects antique medicine bottles and glassware. She keeps colored water in a few as a decorative touch.

This comment is late, I know, but you CAN use it to dye protein fibers like wool and silk. Also nylon, supposedly, although I haven’t tried nylon.

Dampen the yarn or fabric completely with a solution of water and white vinegar. Squeeze out the excess–you just want the fibers to be uniformly moist before you dye.

You can prepare a single-color dye bath by mixing enough HOT water to cover the item with food coloring (you can also use Easter egg dye, icing dyes, colored powdered drinks like Kool-Aid… same principle) and 1-2 cups of vinegar in a large pot. Dump in the fiber and simmer over medium-low heat until the dye is completely absorbed, or until you get the color desired. Stir if needed, but GENTLY: too much agitation will cause protein fibers to full and stick together,

For a painted effect, mix up small amount of dye in a cup, using about 1/2 hot water, 1/2 white vinegar. Put the fiber in a large glass container that will fit in your microwave, and pour the various colors over as desired, or use a sponge or brush for different effects. Microwave the fiber on “High” setting for two minutes, or until the liquid in the bottom of the glass container turns clear. (Keep an eye on it! It’s not likely to burn, but if you’re using a small container, you might get overflow).

For a painted effect

if you live in a cold climate and have heavy snowfalls and children make snowmen, you can mix it up in a spray bottle with water and decorate the snow people.

You can dye boiled eggs. A nice art activity is to write on or decorate hard-boiled eggs with wax crayons. Then you dip the eggs in food colouring (somewhat diluted). The wax of the crayons and the water don’t mix, so the decoration stands out nicely.