Ok....it's 3 am and I have got to finish this cake. I need to make Columbia Blue buttercream and the color that I made looked right at first, but has since turned into a turquoise color. I used sky blue and black. Should I have used delphinium blue instead. I'm going to fix it later, but I want to make sure I don't mess it up again. It's for a graduation cake and I have to get the blue right. Any suggestions?