If you mean on-line sources, you might check www.4x4trails.net or www.colorado4x4.org. But neither of those will give you a complete picture of Colorado trails. The public-submitted trail ratings at 4x4trails.net vary widely.

If you mean books, Chuck Wells books have become the "standard" 4x4 book around here. His Colorado Volume 1 book covers trails south of I-70. His 2nd edition is really good. His books cover a very wide range of difficulty of trails but IMO are especially good for people interested in moderate difficulty trails, but he covers the trails in south-central Colorado very well. You can buy his books on-line at his web site: www.funtreks.com

There are also a couple 4x4 books of Colorado trails by "Massey & Wilson" that are best for people who are most interested in logging roads and just easier or scenic trails.