It just ignore em, and mark like usual party member aggro color.

Another tank aggro should be blue, but for me it just red if I do not hold aggro.

This should be fixed in the latest release. Also under Threat & Highlighting you need to enable the Highlight Mobs on Off-Tanks option if you haven't already.

You such a life saver man, I really dig neatplates.

Any way to manually assign tank unit with some slash command would be nice. It not always correct assigned by default.

I added a new slash command in the latest version(/nptank) that allows you to manage tanks manually. It's really simplistic and simply toggles the currently targeted units assignment between Tank and None, which gets printed in the chat.
However, units assigned as Tank in the raid frames will still override anything you assign manually.

Working like a charm! Thank you so much!