You can buy fondant in many different ways: large tubs, small packages, pure white, colored, you can make your own, whatever strikes your fancy. I typically will buy or make my fondant white and color it for whatever project I am working on. Coloring fondant is easy, but takes a little bit of time and muscle power, depending on how dark the color needs to be.
First things first, gather your materials. You will need: the amount of fondant you desire to color, gel food coloring, and toothpicks (if you don't have squeeze bottle food coloring). Why gel food coloring? It doesn't add much water to the system so your fondant won't get overly sticky. You may also want rubber gloves if you are making a really dark color. I have stained hands for a few days on multiple occasions.
Next, using the toothpick, dab a small amount of food coloring into the middle of the fondant ball. Pick up the ball and stretch fondant from the bottom of the ball over food coloring. Keep stretching the fondant in various directions.
Eventually, your fondant will start looking streaked like the bottom left photo. If a marbled look is what you are going for, stop here. If you are looking for a more unified color keep stretching and pulling until everything is the same color like the bottom right photo.
I was coloring this fondant for the sock monkey cake topper, so this brown did not end up being dark enough. I had to repeat the above steps until I achieved the darker brown I was looking for. Be careful when adding more color, especially right away because gel food coloring gets darker with time.
If you do add too much, simply knead in some uncolored fondant to lighten it up. But, speaking from experience, it is easier to make it darker than to make it lighter.