Here's one for wilton colors...and thank you Diane for the other charts.
courtesy of http://www.wilton.com/cake/icing/colorchart.cfm
Icing Color Chart
When mixing color always mix a small amount of color to experiment. Start with base color and then add very small amounts of secondary color. Be sure to mix enough color for the cakes to be decorated as it is difficult to match an exact color.
ANTIQUE GOLD Add just an extremely small touch of Leaf Green to Golden Yellow
AQUA Sky Blue and Leaf Green
AVOCADO Use Moss Green color
BLACK Our paste color or Royal Blue, Christmas Red, Orange and Lemon
CHARTREUSE 9 parts Lemon Yellow, 1 part Leaf Green
CORAL Creamy peach and a touch of pink or orange and a touch of pink.
FLESH Add just an extremely small touch of Copper to white icing. Ivory can also be used. Light pink with a small amount of brown.
GRAY Add just a touch of Black to white icing.
HUNTER GREEN Kelly Green and a touch of Black
JADE Leaf green, Royal Blue and a touch of Black
LAVENDER Pink and Violet
MARIGOLD Lemon Yellow and Orange
MAROON Burgundy and Red Red
MAUVE Touch of Burgundy with very little Black.
MISTY GREEN Leaf Green, Royal Blue and a touch of Black
MOSS GREEN Our paste color or Violet and Lemon Yellow
MULBERRY Mix Rose with a touch of Royal Blue.
NAVY BLUE Royal Blue and Black
PERIWINKLE Royal Blue and Violet
PLUM Use violet with a touch of Christmas red.
RASPBERRY Pink and Red Red
RUST Orange, Red Red and Brown
SILVER We do not advise attempting to simulate silver color in icing. Instead, add silver leaves or other silver accessories to the cake.
TEAL Use teal paste color or Lemon Yellow and Sky Blue
TURQUOISE Sky Blue and Lemon Yellow
WARM GOLD Use Golden Yellow with just a touch of brown.