Can I untap Nettle Drone in the untap step?
You are correct. All permanents will always untap during the untap step UNLESS a continuous effect stops them from untapping. This continuous effect could come from


its own static ability (e.g. Basalt Monolith),
another object's static ability stops (e.g. Arena of the Ancients), or
a non-static ability (e.g. Frost Lynx).


Nettle Drone's triggered ability lets you untap it when you cast a colourless spell in addition to the normal conditions where it would untap (that is, during your untap step).