Suppose that you have created a beautiful image in GIMP, but one of the colors in the image has turned out not to be exactly what you wanted. Perhaps you have opened a photo that you just took with your digital camera and have found that the subject's eyes are red. One of the simplest ways to correct these problems is to have GIMP find all instances of one color and replace it with another. Although GIMP has no tool to perform this function automatically, you can do it using a combination of the tools that are available.
Launch GIMP using the desktop or Start menu shortcut, and open the image that you want to modify.
Click the "Select by Color Tool" button in the upper right corner of the "Toolbox" window, or press "Shift" and "O" simultaneously.
Click the color in the image that you want to replace. GIMP automatically selects the same color throughout the image.
Click the "Foreground/Background Color" section of the "Toolbox" window, and then click the color that you want to replace the selected color in the image with. Click "OK."
Click the "Bucket Fill Tool" button in the "Toolbox" window, or press "Shift" and "B" simultaneously.
Click the "Fill whole selection" radio button at the bottom of the "Toolbox" window.
Click any of the areas that the "Select by Color" tool highlighted in the image to replace all instances of that color with the color that you selected in Step 4.
When selecting and replacing a color in an image, you will experience the best results with an image that contains few colors.