Within CO, had it not been the CO Supreme Court, I would agree. But if how elections are run is up to the states (you said), then how is it a federal matter? That's what I'm not clear on, given that you said previously it was up to the state(s).

Oh! Sorry, I didn't understand the question.

Just because something is left to the states doesn't mean it won't implicate federal questions. For example, regulation of wills and trusts (estate law) is generally a matter of state law - but if the state does something that is alleged to violate the Fifth Amendment's protection of property rights, then the assertion of that federal claim will make it a "federal question" case for subject matter jurisdiction. Even if the case runs through state courts, if the disposition of the case ultimately turns entirely on a question of federal law, SCOTUS can hear the appeal. For an example of that, the SCOTUS case striking down term limits followed that procedural history: the case went through the Arkansas courts before being heard on appeal by SCOTUS:

en.wikipedia.org - U.S. Term Limits, Inc. v