Thanks Nikhil! 

I added "display/set/contours/node-values yes" before the coloring command and tested it again, but I still got banded contours. I am not sure if there is anything else that I missed. 

For the second one, I found nothing changed with or without setting "display/set/contours/surfaces". The boundaries appear as lines with specific colors, so they seem to be some markers rather than parts of the contour. 

The method mentioned by Rob solved the issue in another way, but I appreciate your help!