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These instructions are for Pages '09, 4.1 (923).
First Save As and give it a file name, in case you change your mind, and want to go back to what it was before you began to make color changes.
Now click on the area of color you want to change. (It could take several tries to select the area you want.)
Choose Inspector, Graphics, Fill, click the color box, and choose another color from the Color tool that opens.
Depending upon the original, you may have to select several areas in order to recolor the entire document.