I know that it has been a while but, I thought that I would share how I did my rainbow effect. I had purple, pink, red, yellow, blue, green, and white buttercream. I put all the colors in piping bags then I did a line of each color up the side of another bag. I put white in the middle of all the colors, white is needed to keep the colors from blending together and making unflattering colors. I don't have pictures, but it came out so pretty.