How do I change RGB colors to their opposites in C?

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C

color

image

RGB

I have a function which should change the colors in an image to their opposites. Red to green, green to blue, and blue to red. I have thinking and trying for a while now, but I can not get it to work. However, I know that there is not any problem with my code. Because the image does change colors, but I have a hard time to make it right.

What I have tried:

This is the code I have for now. I tried to look at a color wheel to see if I was able to move the pixel's color a third of the wheel. Color Wheel Chart Online[^]

My code looks like this:

void reColor(Image *img) {

	img->height = 200;
	img->width = 150;

	for (unsigned int i = 0; i < img->height; i++)
	{
		for (unsigned int j = 0; j < img->width; j++)
		{
			if ((img->pixels[i][j].r <= 255 && img->pixels[i][j].g <= 255)                        && img->pixels[i][j].b == 0){
				while (img->pixels[i][j].r != 0) {
					img->pixels[i][j].r -= 1;
					img->pixels[i][j].g += 1;
				}
			}
			else if ((img->pixels[i][j].r == 0 && img->pixels[i][j].g 
<= 255) && img->pixels[i][j].b <= 255) {
				while (img->pixels[i][j].g != 0) {
					img->pixels[i][j].g -= 1;
					img->pixels[i][j].b += 1;
				}
			}
			else if ((img->pixels[i][j].r <= 255 && img->pixels[i][j].g == 0) && img->pixels[i][j].b <= 255) {
				while (img->pixels[i][j].b != 0) {
					img->pixels[i][j].b -= 1;
					img->pixels[i][j].r += 1;
				}
			}
		}
	}
}