Text filled with color of an image

I'm trying to figure out the process to have text filled in with an image. For instance: letters filled in with the colors of a rainbow in front of a white background. I have recently figured out the basics of layering, but have been unable to pull this technique off. Anybody got an idea?

 

I'm sure this will help you out:
http://www.photoshopessentials.com/photoshop-text/text-effects/image-in-text/

 

yes it is really nice & helpful link. you can also search from youtube as well