Baby Shower Sugar Cookies

I was asked to do cookies for a baby shower.  Because the invitations to the baby shower were turtles, we decided to do turtle cookies.
Invitation and Sugar Cookie
It was fun trying to match the colors – I must admit that one of my weaknesses is mixing colors – experimenting with combining colors to create just the perfect color.  But I guess that will come with practice.   I would love to hear any suggestions or great tips that anyone has regarding mixing colors.  What is your favorite kind of coloring?  Gel paste or powder?