Welcome to Colorado! Enjoy your visit. Please don't stay.
The really super-cool higher up trails are snowed under still, and will be until mid to late July. The backroad paved passes are opening up. Trailridge road is now open (goes thru Rocky Mountain park, and is the highest paved pass in Colorado). Not offroading, but great views, wonderful drive.
I'd recommend traildamage.com as a great site to look at the various trails, reviews and ratings.
Then, I'd recommend Bushducks page
where they help all of us keep up on trail closures and opens.
I'd recommend colorado4x4.org for immediate help from local offroaders. They can be a bit standoffish to newbies, but they will help.
And, once you get a better idea where you're going, let me know, and I can make some suggestions. Colorado is a big state. It's about nine hours from my house to the southwestern corner.