You want to use one of the pre-defined 'output shader' setups. This type of shader loops though all the rendered pixels and changes their color or their position based on the pattern or map used in the shader. To alter just the pixel color, you can use the plain outputStart and outputEnd boxes with a procedural pattern box in the middle. The pixel color can then be multipied by the output of the pattern box and/or another color and then mixed with the original color using the pattern to control which color is choosen.

This is an example of a basic procedural output shader:

The rendered image looks like:

However if you want to move the pixels around in the image, you will need to to use the outputBufferedStart and outputBufferedEnd boxes. The shader needs to buffer the entire image, so it can keep the original pixel color, while assigning another pixel color to that location.

This is an example of a buffered procedural output shader:

The rendered image looks like: