Creatures with "Whenever you cast a colorless spell, untap ~"
One option is to use Scryfall, which uses "~" in card text search to mean the name of the card that the text is on. So, for example, you can search


  o:"Whenever you cast a colorless spell, untap ~"


This gives exactly one result: Nettle Drone.

In addition, Scryfall has a regular expression search that supports the same meaning of "~". So if you do the following card text search, you end up at Nettle Drone:


  re:"Whenever you cast a colorless spell, untap ~"


For a slightly broader set of results, you can search for


  re:"Whenever * (cast|casts) (a|an)* spell,* untap ~"


This finds every ability of the form "Whenever [a player] cast(s) a(n) [characteristic] spell, untap [this permanent]". It finds a total of 9 cards, two of which untap when you cast any colorless spell: Dwarven Patrol and Nettle Drone.

You can also use this less-accurate regex:


  re:"Whenever * (cast|casts) (a|an)* spell,* untap"


This captures more cards outside our intentions but with a similar effect (e.g. Paradox Engine, Jeskai Ascendancy). More importantly, it includes cards like Blistercoil Weird which say "untap it" (in reference to themselves) following some other effect that's also occurring.