function swapRedGreen(pixel) {
    if (pixel.getRed() >= pixel.getGreen() + pixel.getBlue()) {
        pixel.setRed(0);
        pixel.setGreen(255);
    }
    return px;
}

var image = new SimpleImage("...");
print(image);

for (var pixel of image.values()) {
      swapRedGreen();
}
print(image);

1. Work example by hand.
2. Write down what you did.
3. Find patterns.
4. Check by hand.
5. Translate to code.
6. Run test Cases.
7. Debug Failed test Cases.

® Not mine, just steal it as usual.