Yah the graphics hardware these days is `true color` so instead of having an index color like say a palette of 256 colors which you can change and automatically update, you instead store the actual palette value in the pixel, so what you're looking at is like one giant palette. That means even if you have the same color on more than one pixel the pixels are not connected together in any way via the color, so changing the color on one pixel will only change that pixel. As Zawran said you have to manually go through all the pixels in the image, find those which have the color value you want, and then change them to what you want.