How can I create a pixel labelled image for Semantic Segmentation?

I am following the Semantic Segmentation Examples tutorial by MathWorks.

I understand that I can load pixel labeled images

 pxDir = fullfile(dataDir,'buildingPixelLabels');

Define the class names.

classNames = ["sky" "grass" "building" "sidewalk"];

Define the label ID for each class name.

pixelLabelID = [1 2 3 4];

and create a pixelLabelDatastore.

   pxds = pixelLabelDatastore(pxDir,classNames,pixelLabelID);

But, how do I create a custom pixel labelled imagematlab where every pixel value represents the categorical label of that pixel?

I would then proceed by writing:

   pxDir = fullfile(dataDir,'myCustomPixelLabels');

If I understood correctly, `imageDatastore` holds the actual image and not the pixel labels for that image.