Write a function that posterizes an input picture along a very specific process. If the red value is greater than 180, then set the pixel to red. If not, then check if the blue value is greater than 180, and if so, set the pixel to blue. If not, then check if the green value is greater than 180, and if so, set the pixel to green. If none of the three channels is greater than 180, set the pixel to black.