In this case - I agree that Colorado was making the decision for Colorado.
The way I see it - Colorado had the courage to take on this issue and decided for their State.
With Tr**p off the ballot in Colorado - he would have no chance to gain any electoral votes for that State. He still could amass the number of electoral votes in the other States to win the presidency. So Colorado was 'not deciding the election for the nation'. Only for Colorado.
Now if other States had followed through and made the same decision as Colorado to not allow Tr**p on the ballot - they too would be making the decision only for their respective State.
Now if - and only if - enough States joined in and did not allow Tr**p on the ballot - it is only then when it could reach the point where it would be impossible for Tr**p to win enough electoral votes to win the presidency.
But - yes each State would be making a decision for their respective State - States Rights again here in play - however, in my opinion - when it reaches the point in the process where a candidate can't amass the required number of electoral votes to win - as a group - they are now speaking for the majority of the American People that such a candidate shouldn't be elected president.
Now - if SCOTUS votes against Colorado - in fact - they are inhibiting other States to - if the State wanted to - invoke their States Right to eliminate a candidate from the ballot on their State. Kinda like cutting them off at the pass.
Bottom line - SCOTUS is inhibiting the possibility of enough States to speak for the American People.
I hope you could follow my logic here. This is how I'm interpreting this whole matter.