For the National Parks in Colorado, you don't need to make reservations a year in advance. In fact, you can't. There's a 180 day window. If you are outside of that window, The Reserve America site will tell you what date to check back.
For the CG on the east side of Rocky Mountain NP, you will need reservations. On the west side there's Timber Creek CG and you CAN'T make reservations -- it's all first come/first served.
On both sides of Rocky, there's a variety of USFS CG, and if you're there during the week, reservations usually are not needed, but on weekends, you'll need them.
There's also Mesa Verde, Great Sand Dunes, Black Canyon of the Gunnison for National Parks as well as Dinosaur and Colorado National Monuments to visit. Mesa Verde and Great Sand Dunes have CG inside the respective parks, as well as commercial CG outside, and at Great Sand Dunes there's a State Park not far away -- San Luis SP.
Can't advise in Black Canyon of the Gunnison, since I've not camped there. But the others for Mesa Verde and Great Sand Dunes you'll likely need reservations. Again, 180 days out.
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