The FS provides free MVUM's to help everyone know what's a legal trail and what isn't here' a link to them - Stay The Trail Colorado - Where To Go - Forest Service Motor Vehicle Use Maps
There is question as to their accuracy and whether or not they are as current as they should be but they're the best we have to rely on right now.
If you're not familiar with trail ettitquette which includes more than common courtesy you should also read this page and links on the page: Stay The Trail Colorado - Trail Etiquette
There are always gray areas and while we would like to be able to be clear on what we do as within the rules and regulations it's also important to use common sense. For example; if you come upon someone stuck that needs recovery it may require you to go off the trail to do so. IMHO it's always better to provide help in a situation like this than to adhere to the letter of the law.
Summary - Be smart, be safe, be respectful.