Though albaby brought up another point...does SCOTUS even have jurisdiction? He said that how elections are handled is a state matter. So is this even appropriate for a federal court? They already hit the highest state court in Colorado.

Yes, they'll have jurisdiction - because there now exists a live federal "case or controversy" between the parties over the propriety of the Colorado decision.

Let me illustrate by way of an absurd example. Suppose there's a federal law that requires the President to keep a jar of jellybeans on the Resolute Desk under certain circumstances. I, as a random citizen, have no standing to bring a lawsuit to enforce that law if Biden fails to keep a jar of jellybeans on his desk when I think those circumstances have been met. I have no interest or injury at stake to be resolved by a court.

But suppose a federal court lets me file anyway, and issues an order forcing Biden to put jellybeans on his desk. Unlike the initial situation, where I had no actual interest or injury in the case, the analysis turns to whether Biden has an interest or injury in the outcome of that decision. And he clearly does. Biden's interests are concrete, specific to him, and require him (as the aggrieved party) to take an action. So now there is an interest or injury that a party can allege to invoke the jurisdiction of the court.