Image Manipulation Filter

I want to manipulate an image--pixel by pixel--so that when I apply the filter to the image, the resulting image is different from the original.

For one filter, I want to create a watercolor effect to the image. For another, I want to greyscale the image. However, I don't really know how to do either....

I have something like this:

public class EffectFilter implements Filter {
 
  public void filter(PixelImage pi) {
    Pixel[][] data = pi.getData();
   
    for (int row = 0; row < pi.getWidth(); row++) {
      for (int col = 0; col < pi.getHeight(); col++) {
        /** Create an effect for every pixel of the image */
      }
    }
  }
   
This is for an intro java programming class, so I'm a beginner.

Any help appreciated. Thanks!