How do I make turquoise colored frosting?

Wilton sells an icing colour in Teal...which would probably be just fine.

http://www.wilton.com/store/site/product.cfm?id=3E30B2D9-475A-BAC0-5D5C3DB846DFD354&fid=3E33265A-475A-BAC0-597A6ED538D55E2B

Just mix blue and green. I would probably try two parts blue to 1 part green at first.

I own three icing colors, red, blue and yellow. ALL colors can be made by combining these three colors in different proportions and combinations. I find it a lot cheaper to learn how to do this than to have to buy dozens of specific colors to match each specific need.

Teal would be mostly blue, with a very little bit of yellow, but not enough to turn it green. This should bring you to a nice turquoise. now add just the smallest titch of red to cut this down to teal.

To learn how to do this, get a book on color mixing from the Art section of your library. It's easy and will save you a lot of money. Plus it gives you more freedom in that you don't have to be satisfied with the colors that are available...you can adjust until you like it.