I'm big on the candy melts. You could color candy melts or get the colors you want and pipe them out into rainbows. I think it would work. And, if you pipe them onto wax paper and then lightly put another sheet of wax paper on top and lightly press -- both sides will be flat. You just have to be very careful not to press too hard or it will smear.
HTH -- Sarah
Thank you, didnt think about do that!! but how can i make so that the rainbow dont fall, or how can i do to stand the raimbow>>>
Here's two ideas:
1. Right after you make the rainbow or even before, you could put a toothpick or candy stick (depending on how thick you make it and on how big the arc is) and insert it at the bottom of each side of the rainbow. You would then be able to stick that down in the cake and put some RI or BC clouds at the bottom of each rainbow for extra support. Or you could just pipe some BC on the back of the bottom part of the rainbow that touches the cake for support. (Hope this makes sense.)
2. You could make the rainbows bigger (longer) than needed and stick the actual rainbow down into the cake -- and then pipe stuff (clouds, whatever) around the bottom for support.
3. Another idea is to pipe a rainbow in candy melts and then overlay it with fondant. It would add to the strength of it, but would be harder to get the colors right and straight.
Check out my pics for the 50th birthday cake -- the 50 was made from candy melts and used a toothpick for support. I also did this with my son's cake (a giant happy face) that fell over because I only used a toothpick instead of a candy stick.
The airplane cake has candy melts too -- the top wing of the biplane is a 14" long piped candy melt piece wrapped in fondant.
Hope this helps. 
Sarah
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