Any ideas on how to make this rainbow without an airbrush? I will be trying to make a version of this cake for St. Patrick's day. Any ideas will be appreciated.

You could paint it on with a brush, using vodka diluted gel colors.

Would love to try that...

I agree, paint it on. Airbursh colors work, diluted color paste, or you could even use luster dusts.

Thanks for the ideas. Would that work on bc or just fondant?

I would only try that on fondant, maybe you could cut a piece of fondant in white and lay it over buttercream, then just paint the fondant. Good luck! : )

It's certainly easiest on fondant. I have heard of people painting things onto BC though. If it is your first time to paint on a cake I would definately go for the fondant. Use a scrap piece of fondant to practice. The brush you use makes a huge difference in the final look. You can practice on the scrap to decide which brush gives you the desired effect. Good luck!

Thanks everyone!

I've painted on crusting bc with airbrush colors several times. See my cow cake and skate board cakes in my photos. The cow spots and the whole skateboard were done with airbrush colors and a paint brush. You just have to have a light touch while working.

Another idea would be those colors in a spray cake like Wilton or Better Crocker make. You'd have to use something like a paper towel to help give you crisp lines but it could probably be done with them too.

Why don't you try it with strips of fondant? Cut them ti the width you want and lay them all side by side and roll ove rthem with your rolling pin making them one piece and then cut them to the length you need and put it on your cake. HTH