Many MONAI examples use NIFTI format, but you should be able to use NRRD file format just as well. If you have trouble with that then you can post it as a question and @mention some MONAILabel people.
If some of the data is already in NIFTI format then it may be indeed simpler to add more data to it in the same format.
To make things easier for you, I’ve created a color table for the Verse data set. You can download it from here. Simply load this color table then select it when you load a labelmap volume or export a segmentation.