Which picture would you like to use?
Write an equation that selects which pixels you want to change, like x > y which will select all pixels where the x coordinate is greater than the y coordinate.
Then write equations for how to change red, green, and blue (rojo, verde, y azul) for the selected pixels. You can invert each color by subtracting from 255 (e.g., set red/rojo to 255-red (o 255-rojo)).