Hii! How do i make a rainbow for my st pattys cake?? and without using fondant.. any ideas?! thanks =)

Can you do a gel paste transfer? We learned that in Course 1 classes.

Colored piping gels work great
Unless you are wanting this rainbow to stand upright. Then the only way is to pipe it from royal icing and dry (or fill in w/colorflow).

You could use a FBCT, or Royal Icing, I made a St. Pattys day cake this past week, and I used Marshmallow Flow. Add some icing color to it then just heat it in the microwave for about 30 seconds, put into a piping bag and use just like icing. It flows pretty good, you need to use a royal icing as your lines for the design to keep it all in a good shape. It was really fun, and my kids really liked it.

Dang... I just colored some buttercream and using a star tip, piped on a zigzag rainbow... If you are talking about a free standing rainbow, I would think color flow...