Coloring your Icing.

Always start with a clean toothpick when you are using your gel colors. This will make sure that  you do not contaminate the gel.  Start out with a small amount of color and add the color a little at a time until  you  get the desired shade you need. Remember that the colors will darken  even furhter a few hours afer  it sits. Make sure you mix enough of the color you want to ensure that  you do not have to try and match the color with another mixing.

Below are some tips from the Wilton Site. http://www.wilton.com/decorating

 

Mixing Color In Icing

Begin with white icing and use concentrated icing color which will not affect your icing consistency. Using standard food colors can thin down your icing and affect your ability to pipe certain decorations. If you are tinting icing dark brown or black, begin with chocolate icing – your icing will not have the aftertaste that large amounts of these icing colors may produce. If you are tinting a large area red, use No-Taste Red.
Dip a toothpick into the color, then swirl it into the icing. Add color a little at a time until you achieve the shade you desire. Always use a new toothpick each time you add color; you want to avoid getting icing into your jar of color to make it last longer. Blend the icing well with a spatula.
Consider the type of icing you are using when mixing color. Icing colors intensify or darken in buttercream icing about 1-2 hours after mixing. Royal icing requires more color than buttercream icing to achieve the same color intensity.
Always mix enough of any one icing color for your entire cake. For example, if you are going to decorate a cake with pink flowers and borders, color enough icing for both. It is difficult to duplicate an exact shade of any color, an important fact if you want to keep color consistent on the cake.

Bag Striping Effects

You can easily pipe two-tone decorations just by adding a different color inside the bag before you put in your tinted icing. This way, you can pipe flowers with natural light and dark tones or a rainbow-colored clown suit to brighten up the party.

Brush Striping

Produces more intense multiple colors because it is done with straight icing color brushed into the bag. Apply one or more stripes of icing color with a decorating brush, then fill the bag with white or pastel-colored icing. As the icing is squeezed past the color, decorations will come out striped.

Spatula Striping

Produces two-tone and realistic pastel tones in flowers and figure piping. It is done with pastel-colored icing, striped inside the decorating bag with a spatula. After striping, fill the bag with white icing or another shade of the same color as the striping. Squeeze out decorations with soft contrasts.