The advice provided so far is excellent.

20 degree bag is fine for three season Colorado if the temperature rating is accurate.

Down is great. Leave your bag on shore when using your waders.

I use a 30 degree bag for 3 season in Colorado, but I also wear a fleece hoodie as part of my sleep system.

One the rare occasions that I have gotten cold in the backcountry the problem was:

1. Did not eat well enough before bed.

2. Pad did not provide enough insulation.

You are most likely to get cold from the bottom.
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"In theory, theory and practice are the same. In practice, they are not."
Yogi Berra