Hi Cathy -
I am not an Attorney, only a Field Landman in Texas, and suggest that you always consult with an Attorney specializing in Colorado Real Estate law and/or Colorado Mineral Estate law for questions like this.
Having said that, from my experience the cleanest and most above-board way for you to retain your minerals is to simply reserve them in your Deed to whoever buys your property.
I would never advise or advocate "sneakiness" (Moses didn't bring down the Ten Suggestions), but title to mineral interests are not typically covered in a Title Company's review of title for a closing and title insurance purposes. Mineral title research is much, much, much too expensive for typical residential or even commercial real estate transactions.
Rather than scrambling around trying to figure out a way to hide them and then hopeing nobody finds out, just add a provision reserving "...all oil, gas and other minerals in, on and under or that may be produced and saved therefrom..." the land to your deed and don't bring the subject up. If the Buyer brings the subject up, you'll just have to hold the line or negotiate a compromise at that time.
No matter the outcome, at least no one will be ever able to say that you withheld information or tried to "get away with something" during the course of the transaction. I'm from the South and can tell you from truth that a reputation for being "Tacky" will outlive you.
T L Shields is correct: Whatever rights that are not specifically reserved pass forward.
If your Buyer is concerned about oil companies coming on their property, then simply do not reserve the rights of "ingress and egress" [the right to come and go or to enter and to take away], which is typically included as part of a mineral reservation. You could even specify that they pass forward to the new owner if that would make them more at ease. That way, no oil company can come on their land or build a pipeline through their backyard without their permission.
Your idea a transferring your mineral rights to another family member is not particularly a bad one, but before you elect to go that route you should do a little research in the the concepts of Warranty of Title and the Doctrine of After Acquired Title. You can Google or any other Search Engine them - I'm sure you will find more than enough links for them to "learn you up some".
I don't know how Colorado law applies in such circumstances, but under Texas law it is my understanding that you certainly have the right to transfer title to your minerals to another family member or a family trust or any other entitity (Church, Boy Scouts, Children's Hospital Trusts, anything) before your sale to the new owner and that they will then not pass forward.
But also that, under the laws defining Warranty of Title and the Doctrine of After Acquired Title, if the mineral interests are ever re-transferred, re-conveyed or even inherited back into your name, that then and in that event, the mineral interests will immediately pass through you and vest in the new owner.
And because of the "heirs and successors" wording of most documents used to convey title to real property, the effect would be the same whether the interests are conveyed back into your name or into the names of your "heirs and successors" - even 1,000 generations down the line: The mineral interests would immediately pass through and vest in the name of the then owner of the land.
For the Purist out there, I agree that it might be argued that interim reservations might interfere with my simple example here, but I am under the impression that you cannot or should not be able to effectively reserve something you do not own at the time.
Hope this helps -
Charles Emery Tooke III
Certified Professional Landman
Fort Worth, Texas