How do I choose a color from a photo and use it with the fill tool?

I am, for example, posting movie posters that have a frame around them. I want to select a color within the movie poster image and then use the fill tool (the spilling can of paint) to fill the frame around the image with that color. I know how to select a color with the eyedropper from the pallette but when I use the eyedropper to select a color from the image, it won't fill with that color. Thanks for your help.